Messages in this thread | | | Date | Thu, 16 Dec 2021 10:00:09 -0800 | From | Kees Cook <> | Subject | Re: [PATCH 06/17] fortify: Detect struct member overflows in memcpy() at compile-time |
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On Thu, Dec 16, 2021 at 11:08:26AM +0000, Mark Rutland wrote: > On Mon, Dec 13, 2021 at 02:33:20PM -0800, Kees Cook wrote: > > memcpy() is dead; long live memcpy() > > > > tl;dr: In order to eliminate a large class of common buffer overflow > > flaws that continue to persist in the kernel, have memcpy() (under > > CONFIG_FORTIFY_SOURCE) perform bounds checking of the destination struct > > member when they have a known size. This would have caught all of the > > memcpy()-related buffer write overflow flaws identified in at least the > > last three years. > > > > Hi Kees, > > Since there's a *lot* of context below, it's very easy to miss some key details > (e.g. that the compile-time warnings are limited to W=1 builds). It would be > really nice if the summary above could say something like:
Hm, I do need to write a better summary! I think there's still some misunderstanding, and I will attempt some clarity here... :)
> > This patch makes it possible to detect when memcpy() of a struct member may > go past the bounds of that member. When CONFIG_FORTIFY_SOURCE=y, runtime > checks are always emitted where the compiler cannot guarantee a memcpy() is > safely bounded, and compile-time warnings are enabled for W=1 builds.
For GCC and Clang 14, compile-time _write_ overflow warnings are meant to be emitted under FORTIFY_SOURCE. _read_ overflow warnings are meant to be emitted under FORTIFY_SOURCE + W=1 (or when the same statement also has a write overflow).
> > This catches a large class of common buffer overflow flaws, and would have > caught all of the memcpy()-related buffer write overflow flaws identified in > the last three years. > > As an aside, since W=1 is chock-full of (IMO useless) warnings, is there any > way to enable *just* the FORTIFY_SOURCE warnings?
To see them all (i.e. not shove some into W=1), you can remove the "W=1 or write overflow" part of the read overflow test in fortify-string.h. e.g.:
- if ((IS_ENABLED(KBUILD_EXTRA_WARN1) || p_size_field < size) && - q_size_field < size) + if (q_size_field < size)
> I had a go at testing this on arm64, and could get build-time warnings from GCC > 11.1.0, but not from Clang 13.0.0.
This is correct and expected due to Clang 13's lack of support for compiletime_warning().
> No relevant warnings, but code was generated for runtime warnings: > > | 0000000000000000 <foo_copy>: > | 0: d503233f paciasp > | 4: a9bf7bfd stp x29, x30, [sp, #-16]! > | 8: 910003fd mov x29, sp > | c: 52800080 mov w0, #0x4 // #4 > | 10: 52800101 mov w1, #0x8 // #8 > | 14: 94000000 bl 0 <__write_overflow_field> > | 18: 52800080 mov w0, #0x4 // #4 > | 1c: 52800101 mov w1, #0x8 // #8 > | 20: 94000000 bl 0 <__read_overflow2_field> > | 24: 90000008 adrp x8, 8 <foo_copy+0x8> > | 28: f9400108 ldr x8, [x8] > | 2c: 90000009 adrp x9, 0 <foo_copy> > | 30: f9000128 str x8, [x9] > | 34: a8c17bfd ldp x29, x30, [sp], #16 > | 38: d50323bf autiasp > | 3c: d65f03c0 ret > > Have I misunderstood how that's meant to work, or am I doing something wrong?
The generally stated requirement from Linus for these kinds of kernel changes was to never break the build (i.e. we cannot use compiletime_error() -- which Clang 13 falls back to with a link-time failure).
Since this phase of the series is only compile-time warnings (not the run-time warnings), it's rather a no-op for Clang 13. However, the final patch in the series brings the earlier ("mode 0") FORTIFY behaviors to Clang finally.
Clang 14 implements compiletime_warning(), so in that situation, the warnings appear.
It's a pretty wacky Venn Diagram, and I will attempt to include some sort of illustration for it, as the behavioral differences are complex.
-Kees
-- Kees Cook
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