On Tue, Dec 6, 2016 at 10:32 AM, Johannes Thumshirn <jthumshirn@xxxxxxx> wrote: > On Mon, Dec 05, 2016 at 07:03:39PM +0000, Al Viro wrote: >> On Mon, Dec 05, 2016 at 04:17:53PM +0100, Johannes Thumshirn wrote: >> > 633 hp = &srp->header; >> > [...] >> > 646 hp->dxferp = (char __user *)buf + cmd_size; >> >> > So the memory for hp->dxferp comes from: >> > 633 hp = &srp->header; >> >> ???? >> >> > >From my debug instrumentation I see that the dxferp ends up in the >> > iovec_iter's kvec->iov_base and the faulting address is always dxferp + n * >> > 4k with n in [1, 16] (and we're copying 16 4k pages from the iovec into the >> > bio). >> >> _Address_ of hp->dxferp comes from that assignment; the value is 'buf' >> argument of sg_write() + small offset. In this case, it should point >> inside a pipe buffer, which is, indeed, at a kernel address. Who'd >> allocated srp is irrelevant. > > Yes I realized that as well when I had enough distance between me and the > code... > >> >> And if you end up dereferencing more than one page worth there, you do have >> a problem - pipe buffers are not going to be that large. Could you slap >> WARN_ON((size_t)input_size > count); >> right after the calculation of input_size in sg_write() and see if it triggers >> on your reproducer? > > I did and it didn't trigger. What triggers is (as expected) a > WARN_ON((size_t)mxsize > count); > We have count at 80 and mxsize (which ends in hp->dxfer_len) at 65499. But the > 65499 bytes are the len of the data we're suppost to be copying in via the > iov. I'm still rather confused what's happening here, sorry. I think the critical piece here is some kind of race or timing condition. Note that the test program executes all of memfd_create/write/open/sendfile twice. Second time the calls race with each other, but they also can race with the first execution of the calls. -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
