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[v4,3/4] x86/signal: Detect and prevent an alternate signal stack overflow

Message ID 20210115211038.2072-4-chang.seok.bae@intel.com
State Not Applicable
Headers show
Series x86: Improve Minimum Alternate Stack Size | expand

Commit Message

Chang S. Bae Jan. 15, 2021, 9:10 p.m. UTC
The kernel pushes context on to the userspace stack to prepare for the
user's signal handler. When the user has supplied an alternate signal
stack, via sigaltstack(2), it is easy for the kernel to verify that the
stack size is sufficient for the current hardware context.

Check if writing the hardware context to the alternate stack will exceed
it's size. If yes, then instead of corrupting user-data and proceeding with
the original signal handler, an immediate SIGSEGV signal is delivered.

While previous patches in this series allow new source code to discover and
use a sufficient alternate signal stack size, this check is still necessary
to protect binaries with insufficient alternate signal stack size from data

Suggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Reviewed-by: Jann Horn <jannh@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jann Horn <jannh@google.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Changes from v3:
* Updated the changelog (Borislav Petkov)

Changes from v2:
* Simplified the implementation (Jann Horn)
 arch/x86/kernel/signal.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)
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diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 761d856f8ef7..91056a940271 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -242,7 +242,7 @@  get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	unsigned long math_size = 0;
 	unsigned long sp = regs->sp;
 	unsigned long buf_fx = 0;
-	int onsigstack = on_sig_stack(sp);
+	bool onsigstack = on_sig_stack(sp);
 	int ret;
 	/* redzone */
@@ -251,8 +251,11 @@  get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	/* This is the X/Open sanctioned signal stack switching.  */
 	if (ka->sa.sa_flags & SA_ONSTACK) {
-		if (sas_ss_flags(sp) == 0)
+		if (sas_ss_flags(sp) == 0) {
 			sp = current->sas_ss_sp + current->sas_ss_size;
+			/* On the alternate signal stack */
+			onsigstack = true;
+		}
 	} else if (IS_ENABLED(CONFIG_X86_32) &&
 		   !onsigstack &&
 		   regs->ss != __USER_DS &&