From: Dave Hansen dave.hansen@linux.intel.com
commit 74e19ef0ff8061ef55957c3abd71614ef0f42f47 upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/nospec.h | 4 ++++ kernel/bpf/core.c | 2 -- lib/usercopy.c | 7 +++++++ 3 files changed, 11 insertions(+), 2 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 @@ -1567,9 +1567,7 @@ 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 @@ -1,6 +1,7 @@ // SPDX-License-Identifier: GPL-2.0 #include <linux/uaccess.h> #include <linux/bitops.h> +#include <linux/nospec.h>
/* out-of-line parts */
@@ -10,6 +11,12 @@ unsigned long _copy_from_user(void *to, unsigned long res = n; might_fault(); if (likely(access_ok(from, n))) { + /* + * Ensure that bad access_ok() speculation will not + * lead to nasty side effects *after* the copy is + * finished: + */ + barrier_nospec(); kasan_check_write(to, n); res = raw_copy_from_user(to, from, n); }