Messages in this thread | | | From | Jann Horn <> | Date | Tue, 28 Aug 2018 02:51:10 +0200 | Subject | Re: [PATCH v2 6/7] x86: BUG() when uaccess helpers fault on kernel addresses |
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On Mon, Aug 27, 2018 at 8:58 PM Jann Horn <jannh@google.com> wrote: > > There have been multiple kernel vulnerabilities that permitted userspace to > pass completely unchecked pointers through to userspace accessors: > > - the waitid() bug - commit 96ca579a1ecc ("waitid(): Add missing > access_ok() checks") > - the sg/bsg read/write APIs > - the infiniband read/write APIs > > These don't happen all that often, but when they do happen, it is hard to > test for them properly; and it is probably also hard to discover them with > fuzzing. Even when an unmapped kernel address is supplied to such buggy > code, it just returns -EFAULT instead of doing a proper BUG() or at least > WARN(). > > This patch attempts to make such misbehaving code a bit more visible by > refusing to do a fixup in the pagefault handler code when a userspace > accessor causes #PF on a kernel address and the current context isn't > whitelisted.
Kees noticed that when you have vmapped stacks on (I was testing with KASAN, so I couldn't enable vmapped stacks), exact_copy_from_user() in copy_mount_options() is incompatible with this patch when used on kernel pointers. do_mount_root() and devtmpfsd() call kys_mount() with kernel pointers. So I'll have to either add that to the whitelist and/or refactor the mount options copy logic first (so that it uses strncpy_from_user() as long as the data argument isn't binary data).
I've wanted to fiddle with that code anyway - silent truncation at 0xfff bytes isn't very robust, especially when suid binaries call sys_mount() with a partly user-controlled options string...
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