On Mon, Jul 9, 2018 at 8:16 PM Kees Cook [off-list ref] wrote:

On Fri, Jul 6, 2018 at 8:29 PM, NeilBrown [off-list ref] wrote:

quoted

Previous patch ("VFS: simplify seq_file iteration code and interface")
removed code to set ->from to zero when ->count is zero, as ->from is
dead at that time.  However it didn't ensure ->from was set properly
whenever ->count becomes non-zero.
This can only happen when ->show() is called.  Of the three places it
is called one already has ->from set to zero.  The other two are
fixed by setting from to zero after fully flushing the buffer (at which
point ->count will also be zero).

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: NeilBrown <redacted>

I *think* this solves this report, which looks very much like Jann's reproducer:

https://syzkaller.appspot.com/bug?extid=4b712dce5cbce6700f27

I don't think the path I found and what syzcaller reported match
precisely. The syz reproducer linked on that page is:

r0 = memfd_create(&(0x7f00000000c0)='md5sum\x00', 0x0)
mmap(&(0x7f0000001000/0x1000)=nil, 0x1000, 0x0, 0x51, r0, 0x0)
mkdir(&(0x7f0000000040)='./control\x00', 0x0)
r1 = socket$inet_sctp(0x2, 0x1, 0x84)
getsockopt$inet_sctp_SCTP_ADAPTATION_LAYER(r1, 0x84, 0x7,
&(0x7f0000000000), &(0x7f0000000080)=0x4)
r2 = syz_open_procfs(0x0, &(0x7f0000000040)='smaps\x00')
readv(r2, &(0x7f00000021c0)=[{&(0x7f0000000140)=""/79, 0x432},
{&(0x7f00000001c0)=""/4096, 0x1000}], 0x2)

If we assume that this is the same bug, only the last two lines are
interesting. They should cause two invocations of seq_read(). The
first invocation should start with ->from==0, so the bug shouldn't
trigger there. So it has to happen on the second invocation. So the
first invocation has to leave ->from at some high value and ->count
nonzero. But the only place in seq_read() that could set ->from on the
first read (if the traverse case doesn't happen) is the one at the end
of seq_read() that properly adjusts ->count down by the same number in
the line above. This doesn't add up.
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