The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 31255e072b2e91f97645d792d25b2db744186dd1 Gitweb: https://git.kernel.org/tip/31255e072b2e91f97645d792d25b2db744186dd1 Author: Rick Edgecombe rick.p.edgecombe@intel.com AuthorDate: Tue, 07 Nov 2023 10:22:51 -08:00 Committer: Dave Hansen dave.hansen@linux.intel.com CommitterDate: Wed, 08 Nov 2023 08:55:37 -08:00

x86/shstk: Delay signal entry SSP write until after user accesses

When a signal is being delivered, the kernel needs to make accesses to userspace. These accesses could encounter an access error, in which case the signal delivery itself will trigger a segfault. Usually this would result in the kernel killing the process. But in the case of a SEGV signal handler being configured, the failure of the first signal delivery will result in *another* signal getting delivered. The second signal may succeed if another thread has resolved the issue that triggered the segfault (i.e. a well timed mprotect()/mmap()), or the second signal is being delivered to another stack (i.e. an alt stack).

On x86, in the non-shadow stack case, all the accesses to userspace are done before changes to the registers (in pt_regs). The operation is aborted when an access error occurs, so although there may be writes done for the first signal, control flow changes for the signal (regs->ip, regs->sp, etc) are not committed until all the accesses have already completed successfully. This means that the second signal will be delivered as if it happened at the time of the first signal. It will effectively replace the first aborted signal, overwriting the half-written frame of the aborted signal. So on sigreturn from the second signal, control flow will resume happily from the point of control flow where the original signal was delivered.

The problem is, when shadow stack is active, the shadow stack SSP register/MSR is updated *before* some of the userspace accesses. This means if the earlier accesses succeed and the later ones fail, the second signal will not be delivered at the same spot on the shadow stack as the first one. So on sigreturn from the second signal, the SSP will be pointing to the wrong location on the shadow stack (off by a frame).

Pengfei privately reported that while using a shadow stack enabled glibc, the “signal06” test in the LTP test-suite hung. It turns out it is testing the above described double signal scenario. When this test was compiled with shadow stack, the first signal pushed a shadow stack sigframe, then the second pushed another. When the second signal was handled, the SSP was at the first shadow stack signal frame instead of the original location. The test then got stuck as the #CP from the twice incremented SSP was incorrect and generated segfaults in a loop.

Fix this by adjusting the SSP register only after any userspace accesses, such that there can be no failures after the SSP is adjusted. Do this by moving the shadow stack sigframe push logic to happen after all other userspace accesses.

Note, sigreturn (as opposed to the signal delivery dealt with in this patch) has ordering behavior that could lead to similar failures. The ordering issues there extend beyond shadow stack to include the alt stack restoration. Fixing that would require cross-arch changes, and the ordering today does not cause any known test or apps breakages. So leave it as is, for now.

[ dhansen: minor changelog/subject tweak ]

Fixes: 05e36022c054 ("x86/shstk: Handle signals for shadow stack") Reported-by: Pengfei Xu pengfei.xu@intel.com Signed-off-by: Rick Edgecombe rick.p.edgecombe@intel.com Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Tested-by: Pengfei Xu pengfei.xu@intel.com Cc:stable@vger.kernel.org Link: https://lore.kernel.org/all/20231107182251.91276-1-rick.p.edgecombe%40intel.... Link: https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/sysca... --- arch/x86/kernel/signal_64.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/signal_64.c b/arch/x86/kernel/signal_64.c index cacf2ed..23d8aaf 100644 --- a/arch/x86/kernel/signal_64.c +++ b/arch/x86/kernel/signal_64.c @@ -175,9 +175,6 @@ int x64_setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs) frame = get_sigframe(ksig, regs, sizeof(struct rt_sigframe), &fp); uc_flags = frame_uc_flags(regs);

- if (setup_signal_shadow_stack(ksig)) - return -EFAULT; - if (!user_access_begin(frame, sizeof(*frame))) return -EFAULT;

@@ -198,6 +195,9 @@ int x64_setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs) return -EFAULT; }

+ if (setup_signal_shadow_stack(ksig)) + return -EFAULT; + /* Set up registers for signal handler */ regs->di = ksig->sig; /* In case the signal handler was declared without prototypes */