From: Dave Hansen <dave.hansen@linux.intel.com>
Date: Tue, 21 Feb 2023 12:30:15 -0800
Subject: uaccess: Add speculation barrier to copy_from_user()
Git-commit: 74e19ef0ff8061ef55957c3abd71614ef0f42f47
Patch-mainline: v6.3-rc1
References: bsc#1211738 CVE-2023-0459
The results of "access_ok()" can be mis-speculated. The result is that
you can end speculatively:
if (access_ok(from, size))
// Right here
even for bad from/size combinations. On first glance, it would be ideal
to just add a speculation barrier to "access_ok()" so that its results
can never be mis-speculated.
But there are lots of system calls just doing access_ok() via
"copy_to_user()" and friends (example: fstat() and friends). Those are
generally not problematic because they do not _consume_ data from
userspace other than the pointer. They are also very quick and common
system calls that should not be needlessly slowed down.
"copy_from_user()" on the other hand uses a user-controller pointer and
is frequently followed up with code that might affect caches. Take
something like this:
if (!copy_from_user(&kernelvar, uptr, size))
do_something_with(kernelvar);
If userspace passes in an evil 'uptr' that *actually* points to a kernel
addresses, and then do_something_with() has cache (or other)
side-effects, it could allow userspace to infer kernel data values.
Add a barrier to the common copy_from_user() code to prevent
mis-speculated values which happen after the copy.
Also add a stub for architectures that do not define barrier_nospec().
This makes the macro usable in generic code.
Since the barrier is now usable in generic code, the x86 #ifdef in the
BPF code can also go away.
Reported-by: Jordy Zomer <jordyzomer@google.com>
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Daniel Borkmann <daniel@iogearbox.net> # BPF bits
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Michal Koutný <mkoutny@suse.com>
[mkoutny: adjust context, no 3b75c1d884e ("instrumented.h: allow instrumenting both sides of copy_from_user()"), 4.12: no KASAN]
---
include/linux/nospec.h | 4 ++++
kernel/bpf/core.c | 2 --
lib/usercopy.c | 10 +++++++++-
3 files changed, 13 insertions(+), 3 deletions(-)
--- a/include/linux/nospec.h
+++ b/include/linux/nospec.h
@@ -9,6 +9,10 @@
struct task_struct;
+#ifndef barrier_nospec
+# define barrier_nospec() do { } while (0)
+#endif
+
/**
* array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise
* @index: array element index
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1251,9 +1251,7 @@ out:
* reuse preexisting logic from Spectre v1 mitigation that
* happens to produce the required code on x86 for v4 as well.
*/
-#ifdef CONFIG_X86
barrier_nospec();
-#endif
CONT;
#define LDST(SIZEOP, SIZE) \
STX_MEM_##SIZEOP: \
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -1,4 +1,5 @@
#include <linux/uaccess.h>
+#include <linux/nospec.h>
/* out-of-line parts */
@@ -6,8 +7,15 @@
unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n)
{
unsigned long res = n;
- if (likely(access_ok(VERIFY_READ, from, n)))
+ if (likely(access_ok(VERIFY_READ, from, n))) {
+ /*
+ * Ensure that bad access_ok() speculation will not
+ * lead to nasty side effects *after* the copy is
+ * finished:
+ */
+ barrier_nospec();
res = raw_copy_from_user(to, from, n);
+ }
if (unlikely(res))
memset(to + (n - res), 0, res);
return res;