Re: [PATCH] FS: Fixed buffer overflow issue in seq_read()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Nov 18, 2013 at 04:38:03PM -0800, Linus Torvalds wrote:
> Hmm.. Al - this looks like a major oversight, but it also looks like
> the wrong place to initialize count/from in, just because it doesn't
> follow any sane patterns.
> 
> My gut feel is that this needs more cleanup and some sane helper
> function that always initializes those fields when allocating a new
> buffer. Rather than the "initialize in random places and then miss a
> few".
> 
> Afaik, those fields currently get (re-)initialized when:
> 
>  - We do the memset() of the whole seq_file structure at seq_open() time.
> 
>  - at the top of traverse()
> 
>  - count (but not from) gets reinitialized when growing the buffer or
> after traverse() fails in seq_read()
> 
> and it really doesn't give me that happy fuzzy feeling of "that all
> makes sense". Charley's patch seems to fix a missing initialization,
> but I'd *really* like to have it all make more sense, and feel that
> we're not missing some *other* initialization.
> 
> Al?

See upthread.  The bug is real, but I would rather go for a different
fix; it's not worth helper functions, though - we have exactly two places
where free m->buf without freeing m itself, and all we need to do is
clearing m->count in those two places.  No point delaying that to the
next call of seq_read() (and no point cleaning m->from at all), as soon
as we free m->buf we obviously lose all the data that might've been in it.