[PATCH 12/19] x86/signal: Land on &frame->retcode when vdso isn't mapped

[Dmitry Safonov <dima@arista.com>]

Since commit 9fbbd4dd17d0 ("x86: Don't require the vDSO for handling
a.out signals") after processing 32-bit signal if there is no vdso
mapped frame->retcode is used as a landing.
Do the same for rt ia32 signals.

This shouldn't be mistaken for encouragement for running binaries with
executable stack, rather something to do in hopefully very rare
situation with disabled or unmapped vdso and absent SA_RESTORER.
For non-executable stack it'll segfault on attempt to land, rather than
land on a random address where vdso was previously mapped.
For programs with executable stack it'll just do the same for rt signals
as for non-rt.

Discouraging users to run with executable stack is done separately in
commit 47a2ebb7f505 ("execve: warn if process starts with executable
stack").

Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 arch/x86/ia32/ia32_signal.c | 12 +++++++-----
 arch/x86/kernel/signal.c    | 23 ++++++++++-------------
 2 files changed, 17 insertions(+), 18 deletions(-)

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 81cf22398cd1..ea3db15b57bf 100644
--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -270,8 +270,8 @@ int ia32_setup_frame(int sig, struct ksignal *ksig,
 	unsafe_put_user(set->sig[1], &frame->extramask[0], Efault);
 	unsafe_put_user(ptr_to_compat(restorer), &frame->pretcode, Efault);
 	/*
-	 * These are actually not used anymore, but left because some
-	 * gdb versions depend on them as a marker.
+	 * This is popl %eax ; movl $__NR_sigreturn, %eax ; int $0x80
+	 * gdb uses it as a signature to notice signal handler stack frames.
 	 */
 	unsafe_put_user(*((u64 *)&code), (u64 __user *)frame->retcode, Efault);
 	user_access_end();
@@ -336,14 +336,16 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig,
 
 	if (ksig->ka.sa.sa_flags & SA_RESTORER)
 		restorer = ksig->ka.sa.sa_restorer;
-	else
+	else if (current->mm->context.vdso)
 		restorer = current->mm->context.vdso +
 			vdso_image_32.sym___kernel_rt_sigreturn;
+	else
+		restorer = &frame->retcode;
 	unsafe_put_user(ptr_to_compat(restorer), &frame->pretcode, Efault);
 
 	/*
-	 * Not actually used anymore, but left because some gdb
-	 * versions need it.
+	 * This is popl %eax ; movl $__NR_sigreturn, %eax ; int $0x80
+	 * gdb uses it as a signature to notice signal handler stack frames.
 	 */
 	unsafe_put_user(*((u64 *)&code), (u64 __user *)frame->retcode, Efault);
 	unsafe_put_sigcontext32(&frame->uc.uc_mcontext, fp, regs, set, Efault);
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index ea794a083c44..372ec09dc4ac 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -317,23 +317,20 @@ __setup_frame(int sig, struct ksignal *ksig, sigset_t *set,
 	unsafe_put_user(sig, &frame->sig, Efault);
 	unsafe_put_sigcontext(&frame->sc, fp, regs, set, Efault);
 	unsafe_put_user(set->sig[1], &frame->extramask[0], Efault);
-	if (current->mm->context.vdso)
+	if (ksig->ka.sa.sa_flags & SA_RESTORER)
+		restorer = ksig->ka.sa.sa_restorer;
+	else if (current->mm->context.vdso)
 		restorer = current->mm->context.vdso +
 			vdso_image_32.sym___kernel_sigreturn;
 	else
 		restorer = &frame->retcode;
-	if (ksig->ka.sa.sa_flags & SA_RESTORER)
-		restorer = ksig->ka.sa.sa_restorer;
 
 	/* Set up to return from userspace.  */
 	unsafe_put_user(restorer, &frame->pretcode, Efault);
 
 	/*
 	 * This is popl %eax ; movl $__NR_sigreturn, %eax ; int $0x80
-	 *
-	 * WE DO NOT USE IT ANY MORE! It's only left here for historical
-	 * reasons and because gdb uses it as a signature to notice
-	 * signal handler stack frames.
+	 * gdb uses it as a signature to notice signal handler stack frames.
 	 */
 	unsafe_put_user(*((u64 *)&retcode), (u64 *)frame->retcode, Efault);
 	user_access_end();
@@ -382,18 +379,18 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig,
 	unsafe_save_altstack(&frame->uc.uc_stack, regs->sp, Efault);
 
 	/* Set up to return from userspace.  */
-	restorer = current->mm->context.vdso +
-		vdso_image_32.sym___kernel_rt_sigreturn;
 	if (ksig->ka.sa.sa_flags & SA_RESTORER)
 		restorer = ksig->ka.sa.sa_restorer;
+	else if (current->mm->context.vdso)
+		restorer = current->mm->context.vdso +
+			vdso_image_32.sym___kernel_rt_sigreturn;
+	else
+		restorer = &frame->retcode;
 	unsafe_put_user(restorer, &frame->pretcode, Efault);
 
 	/*
 	 * This is movl $__NR_rt_sigreturn, %ax ; int $0x80
-	 *
-	 * WE DO NOT USE IT ANY MORE! It's only left here for historical
-	 * reasons and because gdb uses it as a signature to notice
-	 * signal handler stack frames.
+	 * gdb uses it as a signature to notice signal handler stack frames.
 	 */
 	unsafe_put_user(*((u64 *)&rt_retcode), (u64 *)frame->retcode, Efault);
 	unsafe_put_sigcontext(&frame->uc.uc_mcontext, fp, regs, set, Efault);
-- 
2.28.0