x86: fix error handling for 32-bit compat out-of-range system call numbers

Commit 3f5159a9221f ("x86/asm/entry/32: Update -ENOSYS handling to match
the 64-bit logic") broke the ENOSYS handling for the 32-bit compat case.
The proper error return value was never loaded into %rax, except if
things just happened to go through the audit paths, which ended up
reloading the return value.

This moves the loading or %rax into the normal system call path, just to
make sure the error case triggers it.  It's kind of sad, since it adds a
useless instruction to reload the register to the fast path, but it's
not like that single load from the stack is going to be noticeable.

Reported-by: David Drysdale <drysdale@google.com>
Tested-by: Kees Cook <keescook@chromium.org>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 5a1844765a7a..a7e257d9cb90 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -140,6 +140,7 @@ sysexit_from_sys_call:
          */
         andl    $~TS_COMPAT, ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
         movl    RIP(%rsp), %ecx         /* User %eip */
+        movq    RAX(%rsp), %rax
         RESTORE_RSI_RDI
         xorl    %edx, %edx              /* Do not leak kernel information */
         xorq    %r8, %r8
@@ -219,7 +220,6 @@ sysexit_from_sys_call:
 1:      setbe   %al                     /* 1 if error, 0 if not */
         movzbl  %al, %edi               /* zero-extend that into %edi */
         call    __audit_syscall_exit
-        movq    RAX(%rsp), %rax         /* reload syscall return value */
         movl    $(_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT), %edi
         DISABLE_INTERRUPTS(CLBR_NONE)
         TRACE_IRQS_OFF
@@ -368,6 +368,7 @@ sysretl_from_sys_call:
         RESTORE_RSI_RDI_RDX
         movl    RIP(%rsp), %ecx
         movl    EFLAGS(%rsp), %r11d
+        movq    RAX(%rsp), %rax
         xorq    %r10, %r10
         xorq    %r9, %r9
         xorq    %r8, %r8