• Linus Torvalds's avatar
    x86-64: add warning for non-canonical user access address dereferences · 00c42373
    Linus Torvalds authored
    This adds a warning (once) for any kernel dereference that has a user
    exception handler, but accesses a non-canonical address.  It basically
    is a simpler - and more limited - version of commit 9da3f2b7
    
    
    ("x86/fault: BUG() when uaccess helpers fault on kernel addresses") that
    got reverted.
    
    Note that unlike that original commit, this only causes a warning,
    because there are real situations where we currently can do this
    (notably speculative argument fetching for uprobes etc).  Also, unlike
    that original commit, this _only_ triggers for #GP accesses, so the
    cases of valid kernel pointers that cross into a non-mapped page aren't
    affected.
    
    The intent of this is two-fold:
    
     - the uprobe/tracing accesses really do need to be more careful. In
       particular, from a portability standpoint it's just wrong to think
       that "a pointer is a pointer", and use the same logic for any random
       pointer value you find on the stack. It may _work_ on x86-64, but it
       doesn't necessarily work on other architectures (where the same
       pointer value can be either a kernel pointer _or_ a user pointer, and
       you really need to be much more careful in how you try to access it)
    
       The warning can hopefully end up being a reminder that just any
       random pointer access won't do.
    
     - Kees in particular wanted a way to actually report invalid uses of
       wild pointers to user space accessors, instead of just silently
       failing them. Automated fuzzers want a way to get reports if the
       kernel ever uses invalid values that the fuzzer fed it.
    
       The non-canonical address range is a fair chunk of the address space,
       and with this you can teach syzkaller to feed in invalid pointer
       values and find cases where we do not properly validate user
       addresses (possibly due to bad uses of "set_fs()").
    Acked-by: default avatarKees Cook <keescook@chromium.org>
    Cc: Jann Horn <jannh@google.com>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    00c42373
extable.c 8.69 KB