[SRU][Jammy-OEM-5.17/OEM-6.0][PATCH 1/1] uaccess: Add speculation barrier to copy_from_user()

[Yuxuan Luo]

From: Dave Hansen <dave.hansen at linux.intel.com>

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 at google.com>
Suggested-by: Linus Torvalds <torvalds at linuxfoundation.org>
Signed-off-by: Dave Hansen <dave.hansen at linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx at linutronix.de>
Acked-by: Daniel Borkmann <daniel at iogearbox.net>   # BPF bits
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(backported from commit 74e19ef0ff8061ef55957c3abd71614ef0f42f47)
[yuxuan.luo: There is a conflict at lib/usercopy.c: instrument_copy_from_user()
 requiring commit 33b75c1d884e. This commit was for KMSAN, which is irrelevant
 to this CVE, ignore it.
]
CVE-2023-0459
Signed-off-by: Yuxuan Luo <yuxuan.luo at canonical.com>
---
 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 c1e79f72cd892..9f0af4f116d98 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)
+#endif
+
 /**
  * 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 c4600a5781de1..14a42dcaa7731 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1908,9 +1908,7 @@ 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
 		barrier_nospec();
-#endif
 		CONT;
 #define LDST(SIZEOP, SIZE)						\
 	STX_MEM_##SIZEOP:						\
diff --git a/lib/usercopy.c b/lib/usercopy.c
index 7413dd300516e..7ee63df042d7e 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;
 	might_fault();
 	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(to, from, n);
 		res = raw_copy_from_user(to, from, n);
 	}
-- 
2.34.1