On Wed, Jun 02, 2021 at 11:55:45AM +0200, Thomas Gleixner wrote:

From: Thomas Gleixner tglx@linutronix.de

The non-compacted slowpath uses __copy_from_user() and copies the entire user buffer into the kernel buffer, verbatim. This means that the kernel buffer may now contain entirely invalid state on which XRSTOR will #GP. validate_user_xstate_header() can detect some of that corruption, but that leaves the onus on callers to clear the buffer.

Prior to XSAVES support it was possible just to reinitialize the buffer, completely, but with supervisor states that is not longer possible as the buffer clearing code split got it backwards. Fixing that is possible, but not corrupting the state in the first place is more robust.

Avoid corruption of the kernel XSAVE buffer by using copy_user_to_xstate() which validates the XSAVE header contents before copying the actual states to the kernel. copy_user_to_xstate() was previously only called for compacted-format kernel buffers, but it works for both compacted and non-compacted forms.

Using it for the non-compacted form is slower because of multiple __copy_from_user() operations, but that cost is less important than robust code in an already slow path.

[ Changelog polished by Dave Hansen ]

Nice polishing!

Fixes: b860eb8dce59 ("x86/fpu/xstate: Define new functions for clearing fpregs and xstates") Reported-by: syzbot+2067e764dbcd10721e2e@syzkaller.appspotmail.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: stable@vger.kernel.org

---

arch/x86/include/asm/fpu/xstate.h | 4 ---- arch/x86/kernel/fpu/signal.c | 9 +-------- arch/x86/kernel/fpu/xstate.c | 2 +- 3 files changed, 2 insertions(+), 13 deletions(-)

--- a/arch/x86/include/asm/fpu/xstate.h +++ b/arch/x86/include/asm/fpu/xstate.h @@ -112,8 +112,4 @@ void copy_supervisor_to_kernel(struct xr void copy_dynamic_supervisor_to_kernel(struct xregs_state *xstate, u64 mask); void copy_kernel_to_dynamic_supervisor(struct xregs_state *xstate, u64 mask);

-/* Validate an xstate header supplied by userspace (ptrace or sigreturn) */ -int validate_user_xstate_header(const struct xstate_header *hdr);

#endif --- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -405,14 +405,7 @@ static int __fpu__restore_sig(void __use if (use_xsave() && !fx_only) { u64 init_bv = xfeatures_mask_user() & ~user_xfeatures;

• if (using_compacted_format()) {
  

• 	ret = copy_user_to_xstate(&fpu->state.xsave, buf_fx);
  

• } else {
  

• 	ret = __copy_from_user(&fpu->state.xsave, buf_fx, state_size);
  

• 	if (!ret && state_size > offsetof(struct xregs_state, header))
  

• 		ret = validate_user_xstate_header(&fpu->state.xsave.header);
  

• }
  

• ret = copy_user_to_xstate(&fpu->state.xsave, buf_fx);
  

  if (ret) goto err_out;

--- a/arch/x86/kernel/fpu/xstate.c +++ b/arch/x86/kernel/fpu/xstate.c @@ -515,7 +515,7 @@ int using_compacted_format(void) } /* Validate an xstate header supplied by userspace (ptrace or sigreturn) */ -int validate_user_xstate_header(const struct xstate_header *hdr) +static int validate_user_xstate_header(const struct xstate_header *hdr)

Can't do that yet - that one is still called from regset.c:

arch/x86/kernel/fpu/regset.c: In function ‘xstateregs_set’: arch/x86/kernel/fpu/regset.c:135:10: error: implicit declaration of function ‘validate_user_xstate_header’ [-Werror=implicit-function-declaration] 135 | ret = validate_user_xstate_header(&xsave->header); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~ cc1: some warnings being treated as errors

Maybe after the 5th patch which kills that usage too.

From: Thomas Gleixner tglx@linutronix.de

The non-compacted slowpath uses __copy_from_user() and copies the entire user buffer into the kernel buffer, verbatim. This means that the kernel buffer may now contain entirely invalid state on which XRSTOR will #GP. validate_user_xstate_header() can detect some of that corruption, but that leaves the onus on callers to clear the buffer.

Prior to XSAVES support it was possible just to reinitialize the buffer, completely, but with supervisor states that is not longer possible as the buffer clearing code split got it backwards. Fixing that is possible, but not corrupting the state in the first place is more robust.

Avoid corruption of the kernel XSAVE buffer by using copy_user_to_xstate() which validates the XSAVE header contents before copying the actual states to the kernel. copy_user_to_xstate() was previously only called for compacted-format kernel buffers, but it works for both compacted and non-compacted forms.

Using it for the non-compacted form is slower because of multiple __copy_from_user() operations, but that cost is less important than robust code in an already slow path.

[ Changelog polished by Dave Hansen ]

Fixes: b860eb8dce59 ("x86/fpu/xstate: Define new functions for clearing fpregs and xstates") Reported-by: syzbot+2067e764dbcd10721e2e@syzkaller.appspotmail.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: stable@vger.kernel.org --- V2: Removed the make validate_user_xstate_header() static hunks (Borislav) --- arch/x86/kernel/fpu/signal.c | 9 +-------- 1 file changed, 1 insertion(+), 8 deletions(-)

--- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -405,14 +405,7 @@ static int __fpu__restore_sig(void __use if (use_xsave() && !fx_only) { u64 init_bv = xfeatures_mask_user() & ~user_xfeatures;

- if (using_compacted_format()) { - ret = copy_user_to_xstate(&fpu->state.xsave, buf_fx); - } else { - ret = __copy_from_user(&fpu->state.xsave, buf_fx, state_size); - - if (!ret && state_size > offsetof(struct xregs_state, header)) - ret = validate_user_xstate_header(&fpu->state.xsave.header); - } + ret = copy_user_to_xstate(&fpu->state.xsave, buf_fx); if (ret) goto err_out;