On 11/01/2017 04:29 PM, Kees Cook wrote:

On Wed, Nov 1, 2017 at 4:05 PM, Laura Abbott [off-list ref] wrote:

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On 11/01/2017 03:28 PM, Kees Cook wrote:

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On Wed, Nov 1, 2017 at 2:13 PM, Laura Abbott [off-list ref] wrote:

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On 11/01/2017 05:05 AM, Mark Rutland wrote:

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On Tue, Oct 31, 2017 at 04:56:39PM -0700, Laura Abbott wrote:

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On 10/26/2017 02:09 AM, Mark Rutland wrote:

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In Prague, Kees mentioned that it would be nice to have a mechanism to
catch bad __{get,put}_user uses, such as the recent CVE-2017-5123 [1,2]
issue with unsafe_put_user() in waitid().

These patches allow an optional access_ok() check to be dropped in
arm64's __{get,put}_user() primitives. These will then BUG() if a bad
user pointer is passed (which should only happen in the absence of an
earlier access_ok() check).

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Turning on the option fails as soon as we hit userspace. On my buildroot
based environment I get the help text for ld.so (????) and then a message
about attempting to kill init.

Ouch. Thanks for the report, and sorry about this.

The problem is that I evaluate the ptr argument twice in
__{get,put}_user(), and this may have side effects.

e.g. when the ELF loader does things like:

  __put_user((elf_addr_t)p, sp++)

... we increment sp twice, and write to the wrong user address, leaving
sp corrupt.

I have an additional patch [1] to fix this, which is in my
arm64/access-ok branch [2].

Thanks,
Mark.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/access-ok&id=ebb7ff83eb53b8810395d5cf48712a4ae6d678543
[2] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/access-ok

Thanks, the updated patch works. I wrote an LKDTM test to verify
the expected behavior (__{get,put}_user panic whereas {get,put}_user
do not). You're welcome to add Tested-by or I can wait for v2.

Nice. :) Out of curiosity, can you check if this correctly BUG()s on a
waitid() call when the fixes are reverted?

96ca579a1ecc ("waitid(): Avoid unbalanced user_access_end() on
access_ok() error")
1c9fec470b81 ("waitid(): Add missing access_ok() checks")

-Kees

Yep, we get a nice bug:

[   34.783912] ------------[ cut here ]------------
[   34.784484] kernel BUG at kernel/exit.c:1614!

Awesome! :)

I wonder how hard it might be to make this happen on x86 too (or
generically). Hmmm

x86 looks like it needs the same ptr_argument fixup as arm64 but
seems to have a separate unsafe path so it's actually easier to
fix up. I have version of this that seems to work so I'll clean
it up and send it out tomorrow.

Thanks,
Laura