| Message ID | 20230411022640.8453-1-liangchen.linux@gmail.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | [v3] skbuff: Fix a race between coalescing and releasing SKBs | expand |
On Tue, Apr 11, 2023 at 4:27 AM Liang Chen <liangchen.linux@gmail.com> wrote: > > Commit 1effe8ca4e34 ("skbuff: fix coalescing for page_pool fragment > recycling") allowed coalescing to proceed with non page pool page and page > pool page when @from is cloned, i.e. > > to->pp_recycle --> false > from->pp_recycle --> true > skb_cloned(from) --> true > > However, it actually requires skb_cloned(@from) to hold true until > coalescing finishes in this situation. If the other cloned SKB is > released while the merging is in process, from_shinfo->nr_frags will be > set to 0 toward the end of the function, causing the increment of frag > page _refcount to be unexpectedly skipped resulting in inconsistent > reference counts. Later when SKB(@to) is released, it frees the page > directly even though the page pool page is still in use, leading to > use-after-free or double-free errors. So it should be prohibited. > > Signed-off-by: Liang Chen <liangchen.linux@gmail.com> > --- > Changes from v2: > - switch back to the way v1 works and fix some style issues. > Reviewed-by: Eric Dumazet <edumazet@google.com>
On Tue, 11 Apr 2023 10:26:40 +0800 Liang Chen wrote: > /* In general, avoid mixing slab allocated and page_pool allocated > - * pages within the same SKB. However when @to is not pp_recycle and > - * @from is cloned, we can transition frag pages from page_pool to > - * reference counted. > - * > - * On the other hand, don't allow coalescing two pp_recycle SKBs if > - * @from is cloned, in case the SKB is using page_pool fragment > - * references (PP_FLAG_PAGE_FRAG). Since we only take full page > - * references for cloned SKBs at the moment that would result in > - * inconsistent reference counts. > + * pages within the same SKB. However don't allow coalescing two The word 'however' no longer works here because there's no contradiction, it's an additional rule. > + * pp_recycle SKBs if @from is cloned, in case the SKB is using > + * page_pool fragment references (PP_FLAG_PAGE_FRAG). Since we only > + * take full page references for cloned SKBs at the moment that would > + * result in inconsistent reference counts. > */ > - if (to->pp_recycle != (from->pp_recycle && !skb_cloned(from))) > + if (to->pp_recycle != from->pp_recycle || > + (from->pp_recycle && skb_cloned(from))) How about we change the comment to: /* In general, avoid mixing page_pool and non-page_pool allocated * pages within the same SKB. Additionally avoid dealing with clones * containing page_pool pages, in case the SKB is using page_pool fragment * references (PP_FLAG_PAGE_FRAG). Since we only take full page * references for cloned SKBs at the moment that would result in * inconsistent reference counts. * In theory we could take full references if from is cloned and * !@to->pp_recycle but its tricky (due to potential race with the clone * disappearing) and rare, so not worth dealing with. */ Please also add: Fixes: 53e0961da1c7 ("page_pool: add frag page recycling support in page pool") and Eric's review tag.
diff --git a/net/core/skbuff.c b/net/core/skbuff.c index 050a875d09c5..24a8223f8853 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -5597,17 +5598,14 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from, return false; /* In general, avoid mixing slab allocated and page_pool allocated - * pages within the same SKB. However when @to is not pp_recycle and - * @from is cloned, we can transition frag pages from page_pool to - * reference counted. - * - * On the other hand, don't allow coalescing two pp_recycle SKBs if - * @from is cloned, in case the SKB is using page_pool fragment - * references (PP_FLAG_PAGE_FRAG). Since we only take full page - * references for cloned SKBs at the moment that would result in - * inconsistent reference counts. + * pages within the same SKB. However don't allow coalescing two + * pp_recycle SKBs if @from is cloned, in case the SKB is using + * page_pool fragment references (PP_FLAG_PAGE_FRAG). Since we only + * take full page references for cloned SKBs at the moment that would + * result in inconsistent reference counts. */ - if (to->pp_recycle != (from->pp_recycle && !skb_cloned(from))) + if (to->pp_recycle != from->pp_recycle || + (from->pp_recycle && skb_cloned(from))) return false; if (len <= skb_tailroom(to)) {
Commit 1effe8ca4e34 ("skbuff: fix coalescing for page_pool fragment recycling") allowed coalescing to proceed with non page pool page and page pool page when @from is cloned, i.e. to->pp_recycle --> false from->pp_recycle --> true skb_cloned(from) --> true However, it actually requires skb_cloned(@from) to hold true until coalescing finishes in this situation. If the other cloned SKB is released while the merging is in process, from_shinfo->nr_frags will be set to 0 toward the end of the function, causing the increment of frag page _refcount to be unexpectedly skipped resulting in inconsistent reference counts. Later when SKB(@to) is released, it frees the page directly even though the page pool page is still in use, leading to use-after-free or double-free errors. So it should be prohibited. The double-free error message below prompted us to investigate: BUG: Bad page state in process swapper/1 pfn:0e0d1 page:00000000c6548b28 refcount:-1 mapcount:0 mapping:0000000000000000 index:0x2 pfn:0xe0d1 flags: 0xfffffc0000000(node=0|zone=1|lastcpupid=0x1fffff) raw: 000fffffc0000000 0000000000000000 ffffffff00000101 0000000000000000 raw: 0000000000000002 0000000000000000 ffffffffffffffff 0000000000000000 page dumped because: nonzero _refcount CPU: 1 PID: 0 Comm: swapper/1 Tainted: G E 6.2.0+ Call Trace: <IRQ> dump_stack_lvl+0x32/0x50 bad_page+0x69/0xf0 free_pcp_prepare+0x260/0x2f0 free_unref_page+0x20/0x1c0 skb_release_data+0x10b/0x1a0 napi_consume_skb+0x56/0x150 net_rx_action+0xf0/0x350 ? __napi_schedule+0x79/0x90 __do_softirq+0xc8/0x2b1 __irq_exit_rcu+0xb9/0xf0 common_interrupt+0x82/0xa0 </IRQ> <TASK> asm_common_interrupt+0x22/0x40 RIP: 0010:default_idle+0xb/0x20 Signed-off-by: Liang Chen <liangchen.linux@gmail.com> --- Changes from v2: - switch back to the way v1 works and fix some style issues. --- net/core/skbuff.c | 17 +++++++---------- 1 file changed, 7 insertions(+), 10 deletions(-)