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[2001:1c00:570d:ee00:769d:2f77:d166:4700]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-b97f168c141sm430902766b.40.2026.03.19.05.46.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Mar 2026 05:46:17 -0700 (PDT) From: Amir Goldstein To: Christian Brauner Cc: Al Viro , Miklos Szeredi , Paul Moore , Gao Xiang , linux-security-module@vger.kernel.org, selinux@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-fsdevel@vger.kernel.org, linux-unionfs@vger.kernel.org, syzbot+f34aab278bf5d664e2be@syzkaller.appspotmail.com Subject: [PATCH] fs: allow vfs code to open an O_PATH file with negative dentry Date: Thu, 19 Mar 2026 13:46:16 +0100 Message-ID: <20260319124616.1544415-1-amir73il@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-security-module@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The fields f_mapping, f_wb_err, f_sb_err are irrelevant for O_PATH file. Skip setting them for O_PATH file, so that the O_PATH file could be opened with a negative dentry. This is not something that a user should be able to do, but vfs code, such as ovl_tmpfile() can use this to open a backing O_PATH tmpfile before instantiating the dentry. Reported-by: syzbot+f34aab278bf5d664e2be@syzkaller.appspotmail.com Signed-off-by: Amir Goldstein --- Christian, This patch fixes the syzbot report [1] that the backing_file_user_path_file() patch [2] introduces. This is not the only possible fix, but it is the cleanest one IMO. There is a small risk in introducing a state of an O_PATH file with NULL f_inode, but I (and the bots that I asked) did not find any obvious risk in this state. Note that specifically, the user path inode is accessed via d_inode() and not via file_inode(), which makes this safe for file_user_inode() callers. BTW, I missed this regression with the original patch because I only ran the quick overlayfs sanity test. Now I ran a full quick fstest cycle and verified that the O_TMPFILE test case is covered and that the bug is detected. WDYT? Thanks, Amir. [1] https://syzkaller.appspot.com/bug?extid=f34aab278bf5d664e2be [2] https://lore.kernel.org/linux-fsdevel/20260318131258.1457101-1-amir73il@gmail.com/ fs/open.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/fs/open.c b/fs/open.c index 91f1139591abe..2004a8c0d9c97 100644 --- a/fs/open.c +++ b/fs/open.c @@ -893,9 +893,6 @@ static int do_dentry_open(struct file *f, path_get(&f->f_path); f->f_inode = inode; - f->f_mapping = inode->i_mapping; - f->f_wb_err = filemap_sample_wb_err(f->f_mapping); - f->f_sb_err = file_sample_sb_err(f); if (unlikely(f->f_flags & O_PATH)) { f->f_mode = FMODE_PATH | FMODE_OPENED; @@ -904,6 +901,10 @@ static int do_dentry_open(struct file *f, return 0; } + f->f_mapping = inode->i_mapping; + f->f_wb_err = filemap_sample_wb_err(f->f_mapping); + f->f_sb_err = file_sample_sb_err(f); + if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) { i_readcount_inc(inode); } else if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) { -- 2.53.0