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From: Dave Hansen <>
Date: Tue, 21 Feb 2023 12:30:15 -0800
Subject: [PATCH] uaccess: Add speculation barrier to copy_from_user()
References: bsc#1012628
Patch-mainline: 6.2.1
Git-commit: 74e19ef0ff8061ef55957c3abd71614ef0f42f47

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))

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 <>
Suggested-by: Linus Torvalds <>
Signed-off-by: Dave Hansen <>
Reviewed-by: Thomas Gleixner <>
Acked-by: Daniel Borkmann <>   # BPF bits
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>
Signed-off-by: Jiri Slaby <>
 include/linux/nospec.h | 4 ++++
 kernel/bpf/core.c      | 2 --
 lib/usercopy.c         | 7 +++++++
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/linux/nospec.h b/include/linux/nospec.h
index c1e79f72..9f0af4f1 100644
--- a/include/linux/nospec.h
+++ b/include/linux/nospec.h
@@ -11,6 +11,10 @@
 struct task_struct;
+#ifndef barrier_nospec
+# define barrier_nospec() do { } while (0)
  * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise
  * @index: array element index
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index ba3fff17..430c66d5 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1910,9 +1910,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
 		 * reuse preexisting logic from Spectre v1 mitigation that
 		 * happens to produce the required code on x86 for v4 as well.
-#ifdef CONFIG_X86
 #define LDST(SIZEOP, SIZE)						\
 	STX_MEM_##SIZEOP:						\
diff --git a/lib/usercopy.c b/lib/usercopy.c
index 1505a52f..d29fe29c 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -3,6 +3,7 @@
 #include <linux/fault-inject-usercopy.h>
 #include <linux/instrumented.h>
 #include <linux/uaccess.h>
+#include <linux/nospec.h>
 /* out-of-line parts */
@@ -12,6 +13,12 @@ unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n
 	unsigned long res = n;
 	if (!should_fail_usercopy() && 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();
 		instrument_copy_from_user_before(to, from, n);
 		res = raw_copy_from_user(to, from, n);
 		instrument_copy_from_user_after(to, from, n, res);