| Message ID | 20230317105608.19393-1-jlayton@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [1/2] nfsd: don't replace page in rq_pages if it's a continuation of last page | expand |
On Fri, 2023-03-17 at 06:56 -0400, Jeff Layton wrote: > The splice read calls nfsd_splice_actor to put the pages containing file > data into the svc_rqst->rq_pages array. It's possible however to get a > splice result that only has a partial page at the end, if (e.g.) the > filesystem hands back a short read that doesn't cover the whole page. > > nfsd_splice_actor will plop the partial page into its rq_pages array and > return. Then later, when nfsd_splice_actor is called again, the > remainder of the page may end up being filled out. At this point, > nfsd_splice_actor will put the page into the array _again_ corrupting > the reply. If this is done enough times, rq_next_page will overrun the > array and corrupt the trailing fields -- the rq_respages and > rq_next_page pointers themselves. > > If we've already added the page to the array in the last pass, don't add > it to the array a second time when dealing with a splice continuation. > This was originally handled properly in nfsd_splice_actor, but commit > 91e23b1c3982 removed the check for it, and started universally replacing > pages. > > Fixes: 91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()") > Reported-by: Dario Lesca <d.lesca@solinos.it> > Tested-by: David Critch <dcritch@redhat.com> > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2150630 > Signed-off-by: Jeff Layton <jlayton@kernel.org> > --- > fs/nfsd/vfs.c | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c > index 502e1b7742db..3709ef57d96e 100644 > --- a/fs/nfsd/vfs.c > +++ b/fs/nfsd/vfs.c > @@ -941,8 +941,11 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf, > struct page *last_page; > > last_page = page + (offset + sd->len - 1) / PAGE_SIZE; > - for (page += offset / PAGE_SIZE; page <= last_page; page++) > - svc_rqst_replace_page(rqstp, page); > + for (page += offset / PAGE_SIZE; page <= last_page; page++) { > + /* Only replace page if we haven't already done so */ Note that I think that this was probably the real rationale for the pp[- 1] check that 91e23b1c3982 removed. Given that, maybe we should flesh this comment out a bit more for posterity? /* * When we're splicing from a pipe, it's possible that * we'll get an incomplete page that may be updated on * a later call. Only splice it into rq_pages once. */ > + if (page != *(rqstp->rq_next_page - 1)) > + svc_rqst_replace_page(rqstp, page); > + } > if (rqstp->rq_res.page_len == 0) // first call > rqstp->rq_res.page_base = offset % PAGE_SIZE; > rqstp->rq_res.page_len += sd->len;
> On Mar 17, 2023, at 9:06 AM, Jeff Layton <jlayton@kernel.org> wrote: > > On Fri, 2023-03-17 at 06:56 -0400, Jeff Layton wrote: >> The splice read calls nfsd_splice_actor to put the pages containing file >> data into the svc_rqst->rq_pages array. It's possible however to get a >> splice result that only has a partial page at the end, if (e.g.) the >> filesystem hands back a short read that doesn't cover the whole page. >> >> nfsd_splice_actor will plop the partial page into its rq_pages array and >> return. Then later, when nfsd_splice_actor is called again, the >> remainder of the page may end up being filled out. At this point, >> nfsd_splice_actor will put the page into the array _again_ corrupting >> the reply. If this is done enough times, rq_next_page will overrun the >> array and corrupt the trailing fields -- the rq_respages and >> rq_next_page pointers themselves. >> >> If we've already added the page to the array in the last pass, don't add >> it to the array a second time when dealing with a splice continuation. >> This was originally handled properly in nfsd_splice_actor, but commit >> 91e23b1c3982 removed the check for it, and started universally replacing >> pages. >> >> Fixes: 91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()") >> Reported-by: Dario Lesca <d.lesca@solinos.it> >> Tested-by: David Critch <dcritch@redhat.com> >> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2150630 >> Signed-off-by: Jeff Layton <jlayton@kernel.org> >> --- >> fs/nfsd/vfs.c | 7 +++++-- >> 1 file changed, 5 insertions(+), 2 deletions(-) >> >> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c >> index 502e1b7742db..3709ef57d96e 100644 >> --- a/fs/nfsd/vfs.c >> +++ b/fs/nfsd/vfs.c >> @@ -941,8 +941,11 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf, >> struct page *last_page; >> >> last_page = page + (offset + sd->len - 1) / PAGE_SIZE; >> - for (page += offset / PAGE_SIZE; page <= last_page; page++) >> - svc_rqst_replace_page(rqstp, page); >> + for (page += offset / PAGE_SIZE; page <= last_page; page++) { >> + /* Only replace page if we haven't already done so */ > > Note that I think that this was probably the real rationale for the pp[- > 1] check that 91e23b1c3982 removed. Given that, maybe we should flesh > this comment out a bit more for posterity? > > /* > * When we're splicing from a pipe, it's possible that > * we'll get an incomplete page that may be updated on > * a later call. Only splice it into rq_pages once. > */ The "real" bug here is that the API contract for pipe splicing isn't well defined, so I agree that it's very likely the pp[-1] check was because a splice can call the actor repeatedly for the same page. No one could remember why that check was there. To be clear, if the passed-in page matches the current page in the rqst, we're "extending the current page" rather than avoiding replacement... maybe: /* * Skip page replacement when extending the contents * of the current page. */ In the patch description, would you mention that this case arises if the READ request is not page-aligned? If you resend this patch, please Cc: viro@ . Thanks for chasing this down! >> + if (page != *(rqstp->rq_next_page - 1)) >> + svc_rqst_replace_page(rqstp, page); >> + } >> if (rqstp->rq_res.page_len == 0) // first call >> rqstp->rq_res.page_base = offset % PAGE_SIZE; >> rqstp->rq_res.page_len += sd->len; > > -- > Jeff Layton <jlayton@kernel.org> -- Chuck Lever
On Fri, 2023-03-17 at 14:16 +0000, Chuck Lever III wrote: > > > On Mar 17, 2023, at 9:06 AM, Jeff Layton <jlayton@kernel.org> wrote: > > > > On Fri, 2023-03-17 at 06:56 -0400, Jeff Layton wrote: > > > The splice read calls nfsd_splice_actor to put the pages containing file > > > data into the svc_rqst->rq_pages array. It's possible however to get a > > > splice result that only has a partial page at the end, if (e.g.) the > > > filesystem hands back a short read that doesn't cover the whole page. > > > > > > nfsd_splice_actor will plop the partial page into its rq_pages array and > > > return. Then later, when nfsd_splice_actor is called again, the > > > remainder of the page may end up being filled out. At this point, > > > nfsd_splice_actor will put the page into the array _again_ corrupting > > > the reply. If this is done enough times, rq_next_page will overrun the > > > array and corrupt the trailing fields -- the rq_respages and > > > rq_next_page pointers themselves. > > > > > > If we've already added the page to the array in the last pass, don't add > > > it to the array a second time when dealing with a splice continuation. > > > This was originally handled properly in nfsd_splice_actor, but commit > > > 91e23b1c3982 removed the check for it, and started universally replacing > > > pages. > > > > > > Fixes: 91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()") > > > Reported-by: Dario Lesca <d.lesca@solinos.it> > > > Tested-by: David Critch <dcritch@redhat.com> > > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2150630 > > > Signed-off-by: Jeff Layton <jlayton@kernel.org> > > > --- > > > fs/nfsd/vfs.c | 7 +++++-- > > > 1 file changed, 5 insertions(+), 2 deletions(-) > > > > > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c > > > index 502e1b7742db..3709ef57d96e 100644 > > > --- a/fs/nfsd/vfs.c > > > +++ b/fs/nfsd/vfs.c > > > @@ -941,8 +941,11 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf, > > > struct page *last_page; > > > > > > last_page = page + (offset + sd->len - 1) / PAGE_SIZE; > > > - for (page += offset / PAGE_SIZE; page <= last_page; page++) > > > - svc_rqst_replace_page(rqstp, page); > > > + for (page += offset / PAGE_SIZE; page <= last_page; page++) { > > > + /* Only replace page if we haven't already done so */ > > > > Note that I think that this was probably the real rationale for the pp[- > > 1] check that 91e23b1c3982 removed. Given that, maybe we should flesh > > this comment out a bit more for posterity? > > > > /* > > * When we're splicing from a pipe, it's possible that > > * we'll get an incomplete page that may be updated on > > * a later call. Only splice it into rq_pages once. > > */ > > The "real" bug here is that the API contract for pipe splicing > isn't well defined, so I agree that it's very likely the pp[-1] > check was because a splice can call the actor repeatedly for the > same page. No one could remember why that check was there. > The whole splice API is a minefield. > To be clear, if the passed-in page matches the current page in > the rqst, we're "extending the current page" rather than avoiding > replacement... maybe: > > /* > * Skip page replacement when extending the contents > * of the current page. > */ > Sure, sounds good. > In the patch description, would you mention that this case > arises if the READ request is not page-aligned? > Does it though? I'm not sure that page alignment has that much to do with it. I imagine you can hit this even with aligned I/Os. My guess is the bigger issue is when your storage is doing sub-page-size I/Os under the hood. We end up filling up part of a page from storage and the kernel submits what it has to the pipe and then the next bit comes in and the page is updated for the next actor call. > If you resend this patch, please Cc: viro@ . Thanks for chasing > this down! > Will do. > > > > + if (page != *(rqstp->rq_next_page - 1)) > > > + svc_rqst_replace_page(rqstp, page); > > > + } > > > if (rqstp->rq_res.page_len == 0) // first call > > > rqstp->rq_res.page_base = offset % PAGE_SIZE; > > > rqstp->rq_res.page_len += sd->len; > > > > -- > > Jeff Layton <jlayton@kernel.org> > > -- > Chuck Lever > >
> On Mar 17, 2023, at 10:59 AM, Jeff Layton <jlayton@kernel.org> wrote: > > On Fri, 2023-03-17 at 14:16 +0000, Chuck Lever III wrote: > >> In the patch description, would you mention that this case >> arises if the READ request is not page-aligned? > > Does it though? I'm not sure that page alignment has that much to do > with it. I imagine you can hit this even with aligned I/Os. Maybe, but no-one has actually seen that. The vast majority of reports of this problem are with unaligned I/O, which POSIX OS NFS clients (like the Linux NFS client) usually avoid. I didn't mean to exclude the possibility of hitting this issue in other ways, but simply observing a common way it is hit. -- Chuck Lever
On Fri, 2023-03-17 at 15:04 +0000, Chuck Lever III wrote: > > > On Mar 17, 2023, at 10:59 AM, Jeff Layton <jlayton@kernel.org> wrote: > > > > On Fri, 2023-03-17 at 14:16 +0000, Chuck Lever III wrote: > > > > > In the patch description, would you mention that this case > > > arises if the READ request is not page-aligned? > > > > Does it though? I'm not sure that page alignment has that much to do > > with it. I imagine you can hit this even with aligned I/Os. > > Maybe, but no-one has actually seen that. The vast majority of > reports of this problem are with unaligned I/O, which POSIX OS > NFS clients (like the Linux NFS client) usually avoid. > > I didn't mean to exclude the possibility of hitting this issue > in other ways, but simply observing a common way it is hit. > An unaligned read will consume an extra page, so maybe it just makes it more likely to overrun the array in that case?
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c index 502e1b7742db..3709ef57d96e 100644 --- a/fs/nfsd/vfs.c +++ b/fs/nfsd/vfs.c @@ -941,8 +941,11 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf, struct page *last_page; last_page = page + (offset + sd->len - 1) / PAGE_SIZE; - for (page += offset / PAGE_SIZE; page <= last_page; page++) - svc_rqst_replace_page(rqstp, page); + for (page += offset / PAGE_SIZE; page <= last_page; page++) { + /* Only replace page if we haven't already done so */ + if (page != *(rqstp->rq_next_page - 1)) + svc_rqst_replace_page(rqstp, page); + } if (rqstp->rq_res.page_len == 0) // first call rqstp->rq_res.page_base = offset % PAGE_SIZE; rqstp->rq_res.page_len += sd->len;
The splice read calls nfsd_splice_actor to put the pages containing file data into the svc_rqst->rq_pages array. It's possible however to get a splice result that only has a partial page at the end, if (e.g.) the filesystem hands back a short read that doesn't cover the whole page. nfsd_splice_actor will plop the partial page into its rq_pages array and return. Then later, when nfsd_splice_actor is called again, the remainder of the page may end up being filled out. At this point, nfsd_splice_actor will put the page into the array _again_ corrupting the reply. If this is done enough times, rq_next_page will overrun the array and corrupt the trailing fields -- the rq_respages and rq_next_page pointers themselves. If we've already added the page to the array in the last pass, don't add it to the array a second time when dealing with a splice continuation. This was originally handled properly in nfsd_splice_actor, but commit 91e23b1c3982 removed the check for it, and started universally replacing pages. Fixes: 91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()") Reported-by: Dario Lesca <d.lesca@solinos.it> Tested-by: David Critch <dcritch@redhat.com> Link: https://bugzilla.redhat.com/show_bug.cgi?id=2150630 Signed-off-by: Jeff Layton <jlayton@kernel.org> --- fs/nfsd/vfs.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)