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Date: Tue, 8 Dec 2020 22:44:54 +0000
From: Brooks Davis <brooks@...ebsd.org>
To: Rich Felker <dalias@...c.org>
Cc: Brooks Davis <brooks@...-eyed-alien.net>, musl@...ts.openwall.com
Subject: Re: out-of-bounds reads in strstr
On Tue, Dec 08, 2020 at 02:53:28PM -0500, Rich Felker wrote:
> On Tue, Dec 08, 2020 at 07:39:19PM +0000, Brooks Davis wrote:
> > The strstr implementation contains the following snippet which results
> > in out-of-bounds reads in memchr (we detect them on CHERI because we
> > have byte-granularity bounds of small buffers):
> >
> > /* Fast estimate for MIN(l,63) */
> > size_t grow = l | 63;
> > const unsigned char *z2 = memchr(z, 0, grow);
> >
> > The use of `|` means this is very much not an approximation of
> > `MIN(l,63)`. What is actually intended here? For CheriBSD (via FreeBSD)
> > I need a way to avoid out-of-bounds reads entirely (`MIN(l,63)` does seem
> > to work in simple system-level testing, but given the mismatch it's
> > unclear that's what was intended).
>
> There is no OOB read in strstr here. The overread is in the
> implementation of memchr, which (if you're using the musl version) is
> relying on the ability to overread as long as the address is aligned
> (assuming protection is at boundaries larger than word size). If this
> is a problem, you should use a version of memchr that does not
> overread. Note that, per 7.24.5.1 The memchr function, ??2:
>
> "The implementation shall behave as if it reads the characters
> sequentially and stops as soon as a matching character is found."
>
> so strstr's use of memchr here is perfectly correct/valid without any
> assumption of implementation details, i.e. [at least this part of]
> strstr is portable C.
Ok, I see that. I'll adjust our (musl-derived) memchr to work
correctly in this case.
That being said. I'm still confused by the comment in strstr. `l | 63`
is closer to `MAX(l,63)` than `MIN(l,63)`.
Thanks,
Brooks
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