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[2403:580d:fda1::299]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-82379bfcab8sm10656042b3a.36.2026.01.31.13.08.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 31 Jan 2026 13:08:51 -0800 (PST) Message-ID: <4c37d4e1-e656-48e3-ac80-83c09fe92625@suse.com> Date: Sun, 1 Feb 2026 07:38:44 +1030 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 6.12] btrfs: prevent use-after-free prealloc_file_extent_cluster() To: JP Kobryn , boris@bur.io, clm@fb.com, dsterba@suse.com Cc: linux-btrfs@vger.kernel.org, stable@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260131185335.72204-1-inwardvessel@gmail.com> Content-Language: en-US From: Qu Wenruo Autocrypt: addr=wqu@suse.com; keydata= xsBNBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAHNGFF1IFdlbnJ1byA8d3F1QHN1c2UuY29tPsLAlAQTAQgAPgIbAwULCQgHAgYVCAkKCwIE FgIDAQIeAQIXgBYhBC3fcuWlpVuonapC4cI9kfOhJf6oBQJnEXVgBQkQ/lqxAAoJEMI9kfOh Jf6o+jIH/2KhFmyOw4XWAYbnnijuYqb/obGae8HhcJO2KIGcxbsinK+KQFTSZnkFxnbsQ+VY fvtWBHGt8WfHcNmfjdejmy9si2jyy8smQV2jiB60a8iqQXGmsrkuR+AM2V360oEbMF3gVvim 2VSX2IiW9KERuhifjseNV1HLk0SHw5NnXiWh1THTqtvFFY+CwnLN2GqiMaSLF6gATW05/sEd V17MdI1z4+WSk7D57FlLjp50F3ow2WJtXwG8yG8d6S40dytZpH9iFuk12Sbg7lrtQxPPOIEU rpmZLfCNJJoZj603613w/M8EiZw6MohzikTWcFc55RLYJPBWQ+9puZtx1DopW2jOwE0EWdWB rwEIAKpT62HgSzL9zwGe+WIUCMB+nOEjXAfvoUPUwk+YCEDcOdfkkM5FyBoJs8TCEuPXGXBO Cl5P5B8OYYnkHkGWutAVlUTV8KESOIm/KJIA7jJA+Ss9VhMjtePfgWexw+P8itFRSRrrwyUf E+0WcAevblUi45LjWWZgpg3A80tHP0iToOZ5MbdYk7YFBE29cDSleskfV80ZKxFv6koQocq0 vXzTfHvXNDELAuH7Ms/WJcdUzmPyBf3Oq6mKBBH8J6XZc9LjjNZwNbyvsHSrV5bgmu/THX2n g/3be+iqf6OggCiy3I1NSMJ5KtR0q2H2Nx2Vqb1fYPOID8McMV9Ll6rh8S8AEQEAAcLAfAQY AQgAJgIbDBYhBC3fcuWlpVuonapC4cI9kfOhJf6oBQJnEXWBBQkQ/lrSAAoJEMI9kfOhJf6o cakH+QHwDszsoYvmrNq36MFGgvAHRjdlrHRBa4A1V1kzd4kOUokongcrOOgHY9yfglcvZqlJ qfa4l+1oxs1BvCi29psteQTtw+memmcGruKi+YHD7793zNCMtAtYidDmQ2pWaLfqSaryjlzR /3tBWMyvIeWZKURnZbBzWRREB7iWxEbZ014B3gICqZPDRwwitHpH8Om3eZr7ygZck6bBa4MU o1XgbZcspyCGqu1xF/bMAY2iCDcq6ULKQceuKkbeQ8qxvt9hVxJC2W3lHq8dlK1pkHPDg9wO JoAXek8MF37R8gpLoGWl41FIUb3hFiu3zhDDvslYM4BmzI18QgQTQnotJH8= In-Reply-To: <20260131185335.72204-1-inwardvessel@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 在 2026/2/1 05:23, JP Kobryn 写道: > Users of filemap_lock_folio() need to guard against the situation where > release_folio() has been invoked during reclaim but the folio was > ultimately not removed from the page cache. This patch covers one location > that was overlooked. Affected code has changed as of 6.17, so this patch is > only targeting stable trees prior. > > After acquiring the folio, use set_folio_extent_mapped() to ensure the > folio private state is valid. This is especially important in the subpage > case, where the private field is an allocated struct containing bitmap and > lock data. > > Without this protection, the race below is possible: > > [mm] page cache reclaim path [fs] relocation in subpage mode > shrink_folio_list() > folio_trylock() /* lock acquired */ > filemap_release_folio() > mapping->a_ops->release_folio() > btrfs_release_folio() > __btrfs_release_folio() > clear_folio_extent_mapped() > btrfs_detach_folio_state() > bfs = folio_detach_private(folio) > btrfs_free_folio_state(folio) > kfree(bfs) /* point A */ > > prealloc_file_extent_cluster() > filemap_lock_folio() > folio_try_get() /* inc refcount */ > folio_lock() /* wait for lock */ > > if (...) > ... > else if (!mapping || !__remove_mapping(..)) > /* > * __remove_mapping() returns zero when > * folio_ref_freeze(folio, refcount) fails /* point B */ > */ > goto keep_locked /* folio remains in cache */ > > keep_locked: > folio_unlock(folio) /* lock released */ > > /* lock acquired */ > btrfs_subpage_clear_updodate() > bfs = folio->priv /* use-after-free */ This patch itself and the root cause look good to me. Reviewed-by: Qu Wenruo > > This patch is intended as a minimal fix for backporting to affected > kernels. As of 6.17, a commit [0] replaced the vulnerable > filemap_lock_folio() + btrfs_subpage_clear_uptodate() sequence with > filemap_invalidate_inode() avoiding the race entirely. That commit was part > of a series with a different goal of preparing for large folio support so > backporting may not be straight forward. However I'm not sure if stable tree even accepts non-upstreamed patches. Thus the stable maintainer may ask you the same question as I did before, why not backport the upstream commit 4e346baee95f? If it's lacking the reason why it's a bug fix, I believe you can modify the commit message to include the analyze and the fixes tag. I'm also curious to learn the proper way for such situation. Thanks, Qu > > Signed-off-by: JP Kobryn > Fixes: 9d9ea1e68a05 ("btrfs: subpage: fix relocation potentially overwriting last page data") > > [0] 4e346baee95f ("btrfs: reloc: unconditionally invalidate the page cache for each cluster") > --- > fs/btrfs/relocation.c | 14 ++++++++++++++ > 1 file changed, 14 insertions(+) > > diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c > index 0d5a3846811a..040e8f28b200 100644 > --- a/fs/btrfs/relocation.c > +++ b/fs/btrfs/relocation.c > @@ -2811,6 +2811,20 @@ static noinline_for_stack int prealloc_file_extent_cluster(struct reloc_control > * will re-read the whole page anyway. > */ > if (!IS_ERR(folio)) { > + /* > + * release_folio() could have cleared the folio private data > + * while we were not holding the lock. > + * Reset the mapping if needed so subpage operations can access > + * a valid private folio state. > + */ > + ret = set_folio_extent_mapped(folio); > + if (ret) { > + folio_unlock(folio); > + folio_put(folio); > + > + return ret; > + } > + > btrfs_subpage_clear_uptodate(fs_info, folio, i_size, > round_up(i_size, PAGE_SIZE) - i_size); > folio_unlock(folio);