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Subject[PATCH 4.4 178/193] x86/syscall: Sanitize syscall table de-references under speculation
4.4-stable review patch.  If anyone has any objections, please let me know.


From: Dan Williams <>

(cherry picked from commit 2fbd7af5af8665d18bcefae3e9700be07e22b681)

The syscall table base is a user controlled function pointer in kernel
space. Use array_index_nospec() to prevent any out of bounds speculation.

While retpoline prevents speculating into a userspace directed target it
does not stop the pointer de-reference, the concern is leaking memory
relative to the syscall table base, by observing instruction cache

Reported-by: Linus Torvalds <>
Signed-off-by: Dan Williams <>
Signed-off-by: Thomas Gleixner <>
Cc: Andy Lutomirski <>
Signed-off-by: David Woodhouse <>
[jwang: port to 4.4, no syscall_64]
Signed-off-by: Jack Wang <>
Signed-off-by: Greg Kroah-Hartman <>
arch/x86/entry/common.c | 2 ++
1 file changed, 2 insertions(+)

--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -20,6 +20,7 @@
#include <linux/export.h>
#include <linux/context_tracking.h>
#include <linux/user-return-notifier.h>
+#include <linux/nospec.h>
#include <linux/uprobes.h>

#include <asm/desc.h>
@@ -381,6 +382,7 @@ __always_inline void do_syscall_32_irqs_

if (likely(nr < IA32_NR_syscalls)) {
+ nr = array_index_nospec(nr, IA32_NR_syscalls);
* It's possible that a 32-bit syscall implementation
* takes a 64-bit parameter but nonetheless assumes that

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