Messages in this thread | | | From | Evgenii Stepanov <> | Date | Tue, 25 Jan 2022 08:59:07 -0800 | Subject | Re: [PATCH] arm64: extable: fix null deref in load_unaligned_zeropad. |
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On Tue, Jan 25, 2022 at 7:50 AM Mark Rutland <mark.rutland@arm.com> wrote: > > On Tue, Jan 25, 2022 at 03:23:20PM +0000, Mark Rutland wrote: > > On Fri, Jan 21, 2022 at 06:34:47PM -0800, Evgenii Stepanov wrote: > > > ex_handler_load_unaligned_zeropad extracts the source and data register > > > numbers from the wrong field of the exception table. > > > > Ouch. Did you find this by inspection, or did this show up in testing? > > > > Sorry about this. > > > > I think we should be a little more explicit as to exactly what goes wrong. How > > about: > > > > | In ex_handler_load_unaligned_zeropad() we erroneously extract the data and > > | addr register indices from ex->type rather than ex->data. As ex->type will > > | contain EX_TYPE_LOAD_UNALIGNED_ZEROPAD (i.e. 4): > > | > > | * We'll always treat X0 as the address register, since EX_DATA_REG_ADDR is > > | extracted from bits [9:5]. Thus, we may attempt to dereference an arbitrary > > | address as X0 may hold an arbitary value. > > | > > | * We'll always treat X4 as the data register, since EX_DATA_REG_DATA is > > | extracted from bits [4:0]. Thus we will corrupt X4 and cause arbitrary > > | behaviour within load_unaligned_zeropad() and its caller. > > | > > | Fix this by extracting both values from ex->data as originally intended. > > > > > Fixes: 753b3236 > > > > That should be expanded, e.g. > > > > Fixes: 753b32368705c396 ("arm64: extable: add load_unaligned_zeropad() handler") > > > > With those changes: > > > > Reviewed-by: Mark Rutland <mark.rutland@arm.com> > > Looking again, sicne this isn't jsut a null-deref, can we also rework the > title, something like: > > | arm64: extable: fix load_unaligned_zeropad() reg indices
That's a much better commit message, thank you! I'll upload v2 shortly.
This was found by updating to a newer QEMU that correctly delivers MTE faults from unaligned memory accesses, and triggers this bug reliably during Android boot. I'll add a stack trace to v2.
> > Thanks, > Mark.
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