From e0fbff18bbcee4f07d46bee172803fad63f6f4dd Mon Sep 17 00:00:00 2001 From: Dave Hansen Date: Tue, 21 Feb 2023 12:30:15 -0800 Subject: [PATCH] uaccess: Add speculation barrier to copy_from_user() 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 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 --- include/linux/nospec.h | 4 ++++ lib/usercopy.c | 7 +++++++ 2 files changed, 11 insertions(+) diff --git a/include/linux/nospec.h b/include/linux/nospec.h index 0c5ef54fd4162..207ef2a20e485 100644 --- 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 diff --git a/lib/usercopy.c b/lib/usercopy.c index 3744b2a8e591c..1e99c1baf4ff5 100644 --- a/lib/usercopy.c +++ b/lib/usercopy.c @@ -1,5 +1,6 @@ // SPDX-License-Identifier: GPL-2.0 #include +#include /* out-of-line parts */ @@ -9,6 +10,12 @@ unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n unsigned long res = n; might_fault(); 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(); kasan_check_write(to, n); res = raw_copy_from_user(to, from, n); }