The syscall table base is a user controlled function pointer in kernel
space. Like, 'get_user, use 'MASK_NOSPEC' to prevent any out of bounds
speculation. While retpoline prevents speculating into the user
controlled target it does not stop the pointer de-reference, the concern
is leaking memory relative to the syscall table base.
Reported-by: Linus Torvalds <torva...@linux-foundation.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: "H. Peter Anvin" <h...@zytor.com>
Cc: x...@kernel.org
Cc: Andy Lutomirski <l...@kernel.org>
Signed-off-by: Dan Williams <dan.j.willi...@intel.com>
---
arch/x86/entry/entry_64.S | 2 ++
arch/x86/include/asm/smap.h | 9 ++++++++-
2 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 4f8e1d35a97c..2320017077d4 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -35,6 +35,7 @@
#include <asm/asm.h>
#include <asm/smap.h>
#include <asm/pgtable_types.h>
+#include <asm/smap.h>
#include <asm/export.h>
#include <asm/frame.h>
#include <asm/nospec-branch.h>
@@ -264,6 +265,7 @@ entry_SYSCALL_64_fastpath:
cmpl $__NR_syscall_max, %eax
#endif
ja 1f /* return -ENOSYS (already in
pt_regs->ax) */
+ MASK_NOSPEC %r11 %rax /* sanitize syscall_nr wrt
speculation */
movq %r10, %rcx
/*
diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
index 2b4ad4c6a226..3b5b2cf58dc6 100644
--- a/arch/x86/include/asm/smap.h
+++ b/arch/x86/include/asm/smap.h
@@ -35,7 +35,14 @@
* this directs the cpu to speculate with a NULL ptr rather than
* something targeting kernel memory.
*
- * assumes CF is set from a previous 'cmp TASK_addr_limit, %ptr'
+ * In the syscall entry path it is possible to speculate past the
+ * validation of the system call number. Use MASK_NOSPEC to sanitize the
+ * syscall array index to zero (sys_read) rather than an arbitrary
+ * target.
+ *
+ * assumes CF is set from a previous 'cmp' i.e.:
+ * cmp TASK_addr_limit, %ptr
+ * cmp __NR_syscall_max, %idx
*/
.macro MASK_NOSPEC mask val
sbb \mask, \mask
