Re: [musl] out-of-bounds reads in strstr

[Rich Felker <dalias@libc.org>]

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.

Rich