[PATCH v4 4/5] x86/pkeys: Restore altstack before sigcontext

[Aruna Ramakrishna]

A process can disable access to the alternate signal stack and still
expect signals to be delivered correctly. handle_signal() updates the
PKRU value to enable access to the atlstack, and makes sure that the
value on the sigframe is the user-defined PKRU value so that it is
correctly restored. However, in sigreturn(), restore_altstack() needs
read access to the alt stack. But the PKRU is already restored from the
sigframe (in restore_sigcontext()) which will disable access to the
alt stack, resulting in a SIGSEGV.

Fix this by restoring altstack before restoring PKRU.

Signed-off-by: Aruna Ramakrishna <aruna.ramakrishna@xxxxxxxxxx>
---
 arch/x86/kernel/signal_32.c | 4 ++--
 arch/x86/kernel/signal_64.c | 6 +++---
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c
index 68f2bfd7d6e7..c7a489f0d943 100644
--- a/arch/x86/kernel/signal_32.c
+++ b/arch/x86/kernel/signal_32.c
@@ -160,10 +160,10 @@ SYSCALL32_DEFINE0(rt_sigreturn)
 
 	set_current_blocked(&set);
 
-	if (!ia32_restore_sigcontext(regs, &frame->uc.uc_mcontext))
+	if (restore_altstack32(&frame->uc.uc_stack))
 		goto badframe;
 
-	if (restore_altstack32(&frame->uc.uc_stack))
+	if (!ia32_restore_sigcontext(regs, &frame->uc.uc_mcontext))
 		goto badframe;
 
 	return regs->ax;
diff --git a/arch/x86/kernel/signal_64.c b/arch/x86/kernel/signal_64.c
index 6b189de005b5..2d053333fcf5 100644
--- a/arch/x86/kernel/signal_64.c
+++ b/arch/x86/kernel/signal_64.c
@@ -260,13 +260,13 @@ SYSCALL_DEFINE0(rt_sigreturn)
 
 	set_current_blocked(&set);
 
-	if (!restore_sigcontext(regs, &frame->uc.uc_mcontext, uc_flags))
+	if (restore_altstack(&frame->uc.uc_stack))
 		goto badframe;
 
-	if (restore_signal_shadow_stack())
+	if (!restore_sigcontext(regs, &frame->uc.uc_mcontext, uc_flags))
 		goto badframe;
 
-	if (restore_altstack(&frame->uc.uc_stack))
+	if (restore_signal_shadow_stack())
 		goto badframe;
 
 	return regs->ax;
-- 
2.39.3