[PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read under speculation

[Dan Williams]

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 <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 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