[tip: x86/urgent] x86/uaccess: Zero the 8-byte get_range case on failure on 32-bit

["tip-bot2 for David Gow" <tip-bot2@linutronix.de>]

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

Commit-ID:     dd35a0933269c636635b6af89dc6fa1782791e56
Gitweb:        https://git.kernel.org/tip/dd35a0933269c636635b6af89dc6fa1782791e56
Author:        David Gow <davidgow@google.com>
AuthorDate:    Wed, 31 Jul 2024 15:30:29 +08:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 01 Aug 2024 21:19:10 +02:00

x86/uaccess: Zero the 8-byte get_range case on failure on 32-bit

While zeroing the upper 32 bits of an 8-byte getuser on 32-bit x86 was
fixed by commit 8c860ed825cb ("x86/uaccess: Fix missed zeroing of ia32 u64
get_user() range checking") it was broken again in commit 8a2462df1547
("x86/uaccess: Improve the 8-byte getuser() case").

This is because the register which holds the upper 32 bits (%ecx) is being
cleared _after_ the check_range, so if the range check fails, %ecx is never
cleared.

This can be reproduced with:
./tools/testing/kunit/kunit.py run --arch i386 usercopy

Instead, clear %ecx _before_ check_range in the 8-byte case. This
reintroduces a bit of the ugliness we were trying to avoid by adding
another #ifndef CONFIG_X86_64, but at least keeps check_range from needing
a separate bad_get_user_8 jump.

Fixes: 8a2462df1547 ("x86/uaccess: Improve the 8-byte getuser() case")
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/all/20240731073031.4045579-1-davidgow@google.com
---
 arch/x86/lib/getuser.S | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index a314622..d066aec 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -88,12 +88,14 @@ SYM_FUNC_END(__get_user_4)
 EXPORT_SYMBOL(__get_user_4)
 
 SYM_FUNC_START(__get_user_8)
+#ifndef CONFIG_X86_64
+	xor %ecx,%ecx
+#endif
 	check_range size=8
 	ASM_STAC
 #ifdef CONFIG_X86_64
 	UACCESS movq (%_ASM_AX),%rdx
 #else
-	xor %ecx,%ecx
 	UACCESS movl (%_ASM_AX),%edx
 	UACCESS movl 4(%_ASM_AX),%ecx
 #endif