From: Joerg Roedel jroedel@suse.de
The code in the NMI handler to adjust the #VC handler IST stack is needed in case an NMI hits when the #VC handler is still using its IST stack. But the check for this condition also needs to look if the regs->sp value is trusted, meaning it was not set by user-space. Extend the check to not use regs->sp when the NMI interrupted user-space code or the SYSCALL gap.
Reported-by: Andy Lutomirski luto@kernel.org Fixes: 315562c9af3d5 ("x86/sev-es: Adjust #VC IST Stack on entering NMI handler") Cc: stable@vger.kernel.org # 5.10+ Signed-off-by: Joerg Roedel jroedel@suse.de --- arch/x86/kernel/sev-es.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c index 84c1821819af..0df38b185d53 100644 --- a/arch/x86/kernel/sev-es.c +++ b/arch/x86/kernel/sev-es.c @@ -144,7 +144,9 @@ void noinstr __sev_es_ist_enter(struct pt_regs *regs) old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]);
/* Make room on the IST stack */ - if (on_vc_stack(regs->sp)) + if (on_vc_stack(regs->sp) && + !user_mode(regs) && + !from_syscall_gap(regs)) new_ist = ALIGN_DOWN(regs->sp, 8) - sizeof(old_ist); else new_ist = old_ist - sizeof(old_ist);
On Wed, Feb 17, 2021 at 01:01:42PM +0100, Joerg Roedel wrote:
From: Joerg Roedel jroedel@suse.de
The code in the NMI handler to adjust the #VC handler IST stack is needed in case an NMI hits when the #VC handler is still using its IST stack. But the check for this condition also needs to look if the regs->sp value is trusted, meaning it was not set by user-space. Extend the check to not use regs->sp when the NMI interrupted user-space code or the SYSCALL gap.
Reported-by: Andy Lutomirski luto@kernel.org Fixes: 315562c9af3d5 ("x86/sev-es: Adjust #VC IST Stack on entering NMI handler") Cc: stable@vger.kernel.org # 5.10+ Signed-off-by: Joerg Roedel jroedel@suse.de
arch/x86/kernel/sev-es.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c index 84c1821819af..0df38b185d53 100644 --- a/arch/x86/kernel/sev-es.c +++ b/arch/x86/kernel/sev-es.c @@ -144,7 +144,9 @@ void noinstr __sev_es_ist_enter(struct pt_regs *regs) old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]); /* Make room on the IST stack */
- if (on_vc_stack(regs->sp))
- if (on_vc_stack(regs->sp) &&
!user_mode(regs) &&!from_syscall_gap(regs))
Why not add those checks to on_vc_stack() directly? Because in it, you can say:
on_vc_stack():
/* user mode rSP is not trusted */ if (user_mode()) return false;
/* ditto */ if (ip_within_syscall_gap()) return false;
...
?
On Wed, Feb 17, 2021 at 4:02 AM Joerg Roedel joro@8bytes.org wrote:
From: Joerg Roedel jroedel@suse.de
The code in the NMI handler to adjust the #VC handler IST stack is needed in case an NMI hits when the #VC handler is still using its IST stack. But the check for this condition also needs to look if the regs->sp value is trusted, meaning it was not set by user-space. Extend the check to not use regs->sp when the NMI interrupted user-space code or the SYSCALL gap.
Reported-by: Andy Lutomirski luto@kernel.org Fixes: 315562c9af3d5 ("x86/sev-es: Adjust #VC IST Stack on entering NMI handler") Cc: stable@vger.kernel.org # 5.10+ Signed-off-by: Joerg Roedel jroedel@suse.de
arch/x86/kernel/sev-es.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c index 84c1821819af..0df38b185d53 100644 --- a/arch/x86/kernel/sev-es.c +++ b/arch/x86/kernel/sev-es.c @@ -144,7 +144,9 @@ void noinstr __sev_es_ist_enter(struct pt_regs *regs) old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]);
/* Make room on the IST stack */
if (on_vc_stack(regs->sp))
if (on_vc_stack(regs->sp) &&!user_mode(regs) &&!from_syscall_gap(regs)) new_ist = ALIGN_DOWN(regs->sp, 8) - sizeof(old_ist); else
Can you get rid of the linked list hack while you're at it? This code is unnecessarily convoluted right now, and it seems to be just asking for weird bugs. Just stash the old value in a local variable, please.
Meanwhile, I'm pretty sure I can break this whole scheme if the hypervisor is messing with us. As a trivial example, the sequence SYSCALL gap -> #VC -> NMI -> #VC will go quite poorly. Is this really better than just turning IST off for #VC and documenting that we are not secure against a malicious hypervisor yet?
--Andy
Hi Andy,
On Wed, Feb 17, 2021 at 10:09:46AM -0800, Andy Lutomirski wrote:
Can you get rid of the linked list hack while you're at it? This code is unnecessarily convoluted right now, and it seems to be just asking for weird bugs. Just stash the old value in a local variable, please.
Yeah, the linked list is not really necessary right now, because of the way nested NMI handling works and given that these functions are only used in the NMI handler right now. The whole #VC handling code was written with future requirements in mind, like what is needed when debugging registers get virtualized and #HV gets enabled. Until its clear whether __sev_es_ist_enter/exit() is needed in any of these paths, I'd like to keep the linked list for now. It is more complicated but allows nesting.
Meanwhile, I'm pretty sure I can break this whole scheme if the hypervisor is messing with us. As a trivial example, the sequence SYSCALL gap -> #VC -> NMI -> #VC will go quite poorly.
I don't see how this would break, can you elaborate?
What I think happens is:
SYSCALL gap (RSP is from userspace and untrusted)
-> #VC - Handler on #VC IST stack detects that it interrupted the SYSCALL gap and switches to the task stack.
-> NMI - Now running on NMI IST stack. Depending on whether the stack switch in the #VC handler already happened, the #VC IST entry is adjusted so that a subsequent #VC will not overwrite the interrupted handlers stack frame.
-> #VC - Handler runs on the adjusted #VC IST stack and switches itself back to the NMI IST stack. This is safe wrt. nested NMIs as long as nested NMIs itself are safe.
As a rule of thumb, think of the #VC handler as trying to be a non-IST handler by switching itself to the interrupted stack or the task stack. If it detects that this is not possible (which can't happen right now, but with SNP), it will kill the guest.
Also #VC is currently not safe against #MC, but this is the same as with NMI and #MC. And more care is needed when SNP gets enabled and #VCs can happen in the stack switching/stack adjustment code itself. I will probably just add a check then to kill the guest if an SNP related #VC comes from noinstr code.
Is this really better than just turning IST off for #VC and documenting that we are not secure against a malicious hypervisor yet?
It needs to be IST, even without SNP, because #DB is IST too. When the hypervisor intercepts #DB then any #DB exception will be turned into #VC, so #VC needs to be handled anywhere a #DB can happen.
And with SNP we need to be able to at least detect a malicious HV so we can reliably kill the guest. Otherwise the HV could potentially take control over the guest's execution flow and make it reveal its secrets.
Regards,
Joerg
On Thu, Feb 18, 2021 at 3:25 AM Joerg Roedel joro@8bytes.org wrote:
Hi Andy,
On Wed, Feb 17, 2021 at 10:09:46AM -0800, Andy Lutomirski wrote:
Can you get rid of the linked list hack while you're at it? This code is unnecessarily convoluted right now, and it seems to be just asking for weird bugs. Just stash the old value in a local variable, please.
Yeah, the linked list is not really necessary right now, because of the way nested NMI handling works and given that these functions are only used in the NMI handler right now. The whole #VC handling code was written with future requirements in mind, like what is needed when debugging registers get virtualized and #HV gets enabled. Until its clear whether __sev_es_ist_enter/exit() is needed in any of these paths, I'd like to keep the linked list for now. It is more complicated but allows nesting.
I don't understand what this means. The whole entry mechanism on x86 is structured so that we call a C function *and return from that C function without longjmp-like magic* with the sole exception of unwind_stack_do_exit(). This means that you can match up enters and exits, and that unwind_stack_do_exit() needs to unwind correctly. In the former case, it's normal C and we can use normal local variables. In the latter case, we know exactly what state we're trying to restore and we can restore it directly without any linked lists or similar.
What do you have in mind that requires a linked list?
Meanwhile, I'm pretty sure I can break this whole scheme if the hypervisor is messing with us. As a trivial example, the sequence SYSCALL gap -> #VC -> NMI -> #VC will go quite poorly.
I don't see how this would break, can you elaborate?
What I think happens is:
SYSCALL gap (RSP is from userspace and untrusted)
-> #VC - Handler on #VC IST stack detects that it interrupted the SYSCALL gap and switches to the task stack.
Can you point me to exactly what code you're referring to? I spent a while digging through the code and macro tangle and I can't find this.
-> NMI - Now running on NMI IST stack. Depending on whether the stack switch in the #VC handler already happened, the #VC IST entry is adjusted so that a subsequent #VC will not overwrite the interrupted handlers stack frame. -> #VC - Handler runs on the adjusted #VC IST stack and switches itself back to the NMI IST stack. This is safe wrt. nested NMIs as long as nested NMIs itself are safe.As a rule of thumb, think of the #VC handler as trying to be a non-IST handler by switching itself to the interrupted stack or the task stack. If it detects that this is not possible (which can't happen right now, but with SNP), it will kill the guest.
I will try to think of this, but it's hard, since I can't find the code :)
I found this comment:
* With the current implementation it is always possible to switch to a safe * stack because #VC exceptions only happen at known places, like intercepted * instructions or accesses to MMIO areas/IO ports. They can also happen with * code instrumentation when the hypervisor intercepts #DB, but the critical * paths are forbidden to be instrumented, so #DB exceptions currently also * only happen in safe places.
Unless AMD is more magic than I realize, the MOV SS bug^Wfeature means that #DB is *not* always called in safe places.
But I *thnk* the code you're talking about is this:
/* * If the entry is from userspace, switch stacks and treat it as * a normal entry. */ testb $3, CS-ORIG_RAX(%rsp) jnz .Lfrom_usermode_switch_stack_@
which does not run on #VC from kernel code.
It needs to be IST, even without SNP, because #DB is IST too. When the hypervisor intercepts #DB then any #DB exception will be turned into #VC, so #VC needs to be handled anywhere a #DB can happen.
Eww.
And with SNP we need to be able to at least detect a malicious HV so we can reliably kill the guest. Otherwise the HV could potentially take control over the guest's execution flow and make it reveal its secrets.
True. But is the rest of the machinery to be secure against EFLAGS.IF violations and such in place yet?
Regards,
Joerg
On Thu, Feb 18, 2021 at 09:49:06AM -0800, Andy Lutomirski wrote:
I don't understand what this means. The whole entry mechanism on x86 is structured so that we call a C function *and return from that C function without longjmp-like magic* with the sole exception of unwind_stack_do_exit(). This means that you can match up enters and exits, and that unwind_stack_do_exit() needs to unwind correctly. In the former case, it's normal C and we can use normal local variables. In the latter case, we know exactly what state we're trying to restore and we can restore it directly without any linked lists or similar.
Okay, the unwinder will likely get confused by this logic.
What do you have in mind that requires a linked list?
Cases when there are multiple IST vectors besides NMI that can hit while the #VC handler is still on its own IST stack. #MCE comes to mind, but that is broken anyway. At some point #VC itself will be one of them, but when that happens the code will kill the machine. This leaves #HV in the list, and I am not sure how that is going to be handled yet. I think the goal is that the #HV handler is not allowed to cause any #VC exception. In that case the linked-list logic will not be needed.
I don't see how this would break, can you elaborate?
What I think happens is:
SYSCALL gap (RSP is from userspace and untrusted)
-> #VC - Handler on #VC IST stack detects that it interrupted the SYSCALL gap and switches to the task stack.Can you point me to exactly what code you're referring to? I spent a while digging through the code and macro tangle and I can't find this.
See the entry code in arch/x86/entry/entry_64.S, macro idtentry_vc. It creates the assembly code for the handler. At some point it calls vc_switch_off_ist(), which is a C function in arch/x86/kernel/traps.c. This function tries to find a new stack for the #VC handler.
The first thing it does is checking whether the exception came from the SYSCALL gap and just uses the task stack in that case.
Then it will check for other kernel stacks which are safe to switch to. If that fails it uses the fall-back stack (VC2), which will direct the handler to a separate function which, for now, just calls panic(). Not safe are the entry or unknown stacks.
The function then copies pt_regs and returns the new stack pointer to assembly code, which then writes it to %RSP.
Unless AMD is more magic than I realize, the MOV SS bug^Wfeature means that #DB is *not* always called in safe places.
You are right, forgot about this. The MOV SS bug can very well trigger a #VC(#DB) exception from the syscall gap.
And with SNP we need to be able to at least detect a malicious HV so we can reliably kill the guest. Otherwise the HV could potentially take control over the guest's execution flow and make it reveal its secrets.
True. But is the rest of the machinery to be secure against EFLAGS.IF violations and such in place yet?
Not sure what you mean by EFLAGS.IF violations, probably enabling IRQs while in the #VC handler? The #VC handler _must_ _not_ enable IRQs anywhere in its call-path. If that ever happens it is a bug.
Regards,
Joerg
On Thu, Feb 18, 2021 at 11:21 AM Joerg Roedel jroedel@suse.de wrote:
On Thu, Feb 18, 2021 at 09:49:06AM -0800, Andy Lutomirski wrote:
I don't understand what this means. The whole entry mechanism on x86 is structured so that we call a C function *and return from that C function without longjmp-like magic* with the sole exception of unwind_stack_do_exit(). This means that you can match up enters and exits, and that unwind_stack_do_exit() needs to unwind correctly. In the former case, it's normal C and we can use normal local variables. In the latter case, we know exactly what state we're trying to restore and we can restore it directly without any linked lists or similar.
Okay, the unwinder will likely get confused by this logic.
What do you have in mind that requires a linked list?
Cases when there are multiple IST vectors besides NMI that can hit while the #VC handler is still on its own IST stack. #MCE comes to mind, but that is broken anyway. At some point #VC itself will be one of them, but when that happens the code will kill the machine. This leaves #HV in the list, and I am not sure how that is going to be handled yet. I think the goal is that the #HV handler is not allowed to cause any #VC exception. In that case the linked-list logic will not be needed.
Can you give me an example, even artificial, in which the linked-list logic is useful?
I don't see how this would break, can you elaborate?
What I think happens is:
SYSCALL gap (RSP is from userspace and untrusted)
-> #VC - Handler on #VC IST stack detects that it interrupted the SYSCALL gap and switches to the task stack.Can you point me to exactly what code you're referring to? I spent a while digging through the code and macro tangle and I can't find this.
See the entry code in arch/x86/entry/entry_64.S, macro idtentry_vc. It creates the assembly code for the handler. At some point it calls vc_switch_off_ist(), which is a C function in arch/x86/kernel/traps.c. This function tries to find a new stack for the #VC handler.
The first thing it does is checking whether the exception came from the SYSCALL gap and just uses the task stack in that case.
Then it will check for other kernel stacks which are safe to switch to. If that fails it uses the fall-back stack (VC2), which will direct the handler to a separate function which, for now, just calls panic(). Not safe are the entry or unknown stacks.
Can you explain your reasoning in considering the entry stack unsafe? It's 4k bytes these days.
You forgot about entry_SYSCALL_compat.
Your 8-byte alignment is confusing to me. In valid kernel code, SP should be 8-byte-aligned already, and, if you're trying to match architectural behavior, the CPU aligns to 16 bytes.
We're not robust against #VC, NMI in the #VC prologue before the magic stack switch, and a new #VC in the NMI prologue. Nor do we appear to have any detection of the case where #VC nests directly inside its own prologue. Or did I miss something else here?
If we get NMI and get #VC in the NMI *asm*, the #VC magic stack switch looks like it will merrily run itself in the NMI special-stack-layout section, and that sounds really quite bad.
The function then copies pt_regs and returns the new stack pointer to assembly code, which then writes it to %RSP.
Unless AMD is more magic than I realize, the MOV SS bug^Wfeature means that #DB is *not* always called in safe places.
You are right, forgot about this. The MOV SS bug can very well trigger a #VC(#DB) exception from the syscall gap.
And with SNP we need to be able to at least detect a malicious HV so we can reliably kill the guest. Otherwise the HV could potentially take control over the guest's execution flow and make it reveal its secrets.
True. But is the rest of the machinery to be secure against EFLAGS.IF violations and such in place yet?
Not sure what you mean by EFLAGS.IF violations, probably enabling IRQs while in the #VC handler? The #VC handler _must_ _not_ enable IRQs anywhere in its call-path. If that ever happens it is a bug.
I mean that, IIRC, a malicious hypervisor can inject inappropriate vectors at inappropriate times if the #HV mechanism isn't enabled. For example, it could inject a page fault or an interrupt in a context in which we have the wrong GSBASE loaded.
But the #DB issue makes this moot. We have to use IST unless we turn off SCE. But I admit I'm leaning toward turning off SCE until we have a solution that seems convincingly robust.
On Thu, Feb 18, 2021 at 04:28:36PM -0800, Andy Lutomirski wrote:
On Thu, Feb 18, 2021 at 11:21 AM Joerg Roedel jroedel@suse.de wrote: Can you give me an example, even artificial, in which the linked-list logic is useful?
So here we go, its of course artificial, but still:
1. #VC happens, not important where 2. NMI in the #VC prologue before it moved off its IST stack - first VC IST adjustment happening here 3. #VC in the NMI handler 4. #HV in the #VC prologue again - second VC IST adjustment happening here, so the #HV handler can cause its own #VC exceptions.
Can only happen if the #HV handler is allowed to cause #VC exceptions. But even if its not allowed, it can happen with SNP and a malicious Hypervisor. But in this case the only option is to reliably panic.
Can you explain your reasoning in considering the entry stack unsafe? It's 4k bytes these days.
I wasn't aware that it is 4k in size now. I still thought it was just these 64 words large and one can not simply execute C code on it.
You forgot about entry_SYSCALL_compat.
Right, thanks for pointing this out.
Your 8-byte alignment is confusing to me. In valid kernel code, SP should be 8-byte-aligned already, and, if you're trying to match architectural behavior, the CPU aligns to 16 bytes.
Yeah, I was just being cautious. The explicit alignment can be removed, Boris also pointed this out.
We're not robust against #VC, NMI in the #VC prologue before the magic stack switch, and a new #VC in the NMI prologue. Nor do we appear to have any detection of the case where #VC nests directly inside its own prologue. Or did I miss something else here?
No, you don't miss anything here. At the moment #VC can't happen at those places, so this is not handled yet. With SNP it can happen and needs to be handled in a way to at least allow a reliable panic (because if it really happens the Hypervisor is messing with us).
If we get NMI and get #VC in the NMI *asm*, the #VC magic stack switch looks like it will merrily run itself in the NMI special-stack-layout section, and that sounds really quite bad.
Yes, I havn't looked at the details yet, but if a #VC happens there it probably better not returns.
I mean that, IIRC, a malicious hypervisor can inject inappropriate vectors at inappropriate times if the #HV mechanism isn't enabled. For example, it could inject a page fault or an interrupt in a context in which we have the wrong GSBASE loaded.
Yes, a malicious Hypervisor can do that, and without #HV there is no real protection against this besides turning all vectors (even IRQs) into paranoid entries. Maybe even more care is needed, but I think its not worth to care about this.
But the #DB issue makes this moot. We have to use IST unless we turn off SCE. But I admit I'm leaning toward turning off SCE until we have a solution that seems convincingly robust.
Turning off SCE might be tempting, but I guess doing so would break a quite some user-space code, no?
Regards,
Joerg
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