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In-Reply-To: <20261021-work-mount-statmount_listns_fixes-v1-1-8917a4c1bd14@kernel.org> References: <20261021-work-mount-statmount_listns_fixes-v1-0-8917a4c1bd14@kernel.org> <20261021-work-mount-statmount_listns_fixes-v1-1-8917a4c1bd14@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ cDEsvv5IMmeMe1/qLZ6NaVkNuL3WOXvxaVT9USW1+/SGipO2IpKJjeDZfehlB/kpfF24+RrK+seQf 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Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.2 (3.60.2-2.fc44) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 On Wed, 2026-10-21 at 13:46 +0200, Christian Brauner wrote: > statmount_string() reserves an empty string at the start of the seq > buffer before it calls the handler for the requested string. That's > what the offsets of unset strings point to since commit e52e97f09fb6 > ("statmount: let unset strings be empty"). >=20 > If the handler emits nothing the early return from commit 75ead69a7173 > ("fs: don't let statmount return empty strings") skips the size check > against the caller's buffer. Back then an empty emission meant an empty > seq buffer and there was nothing to copy. Now the reserved NUL byte is > there and copy_statmount_to_user() copies it to buf + sizeof(struct > statmount) no matter how small bufsize is. >=20 > So request a string that may turn out empty with a buffer that's > smaller than struct statmount and the kernel writes one zero byte past > the caller's buffer and reports a size that's larger than the buffer. > STATMOUNT_MNT_UIDMAP or STATMOUNT_MNT_GIDMAP on a mount that isn't > idmapped will do, so will STATMOUNT_FS_SUBTYPE on anything but fuse or > STATMOUNT_OPT_ARRAY on a filesystem without ->show_options(). None of > this needs privileges. The byte only lands in the caller's own address > space but that's still not ok. >=20 > prepare_kstatmount() already refuses a buffer of exactly sizeof(struct > statmount) for string requests. A smaller buffer can't hold a string > either and it can't even hold the reserved empty string. So refuse any > buffer that isn't larger than the fixed part. That's also what the uapi > header says: string requests need space after the fixed part of the > structure. >=20 > Reproducer, unprivileged, on any mount that isn't idmapped: >=20 > #define _GNU_SOURCE > #include > #include > #include > #include > #include > #include > #include > #include > #define STATMOUNT_FS_SUBTYPE 0x00000100U > #define STATMOUNT_MNT_UIDMAP 0x00002000U > #define STATX_MNT_ID_UNIQUE 0x00004000U > #define SM_SIZE 512 /* sizeof(struct statmount) */ > struct mnt_id_req { > uint32_t size, mnt_ns_fd; > uint64_t mnt_id, param, mnt_ns_id; > }; > static uint64_t root_mnt_id(void) > { > unsigned char stx[256]; /* struct statx, stx_mnt_id at offset 144 */ > uint64_t id; > if (syscall(__NR_statx, AT_FDCWD, "/", 0, STATX_MNT_ID_UNIQUE, stx)) > _exit(1); > memcpy(&id, stx + 144, sizeof(id)); > return id; > } > static void probe(uint64_t mnt_id, uint64_t mask, int guard) > { > struct mnt_id_req req =3D { > .size =3D sizeof(req), .mnt_id =3D mnt_id, .param =3D mask, > }; > long page =3D sysconf(_SC_PAGESIZE); > unsigned char *map, *buf; > uint32_t size; > long ret; > map =3D mmap(NULL, 2 * page, PROT_READ | PROT_WRITE, > MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > memset(map, 0xaa, 2 * page); > /* 100 byte buffer ending right at the page boundary */ > buf =3D map + page - 100; > if (guard) > mprotect(map + page, page, PROT_NONE); > errno =3D 0; > ret =3D syscall(__NR_statmount, &req, buf, 100, 0); > printf("mask 0x%04llx%s: ret %ld errno %d", (unsigned long long)mask= , > guard ? " guard page" : "", ret, errno); > if (!guard) { > memcpy(&size, buf, sizeof(size)); > printf(", size 0x%x, byte[%d] 0x%02x", size, SM_SIZE, > buf[SM_SIZE]); > } > printf("\n"); > munmap(map, 2 * page); > } > int main(void) > { > uint64_t id =3D root_mnt_id(); > probe(id, STATMOUNT_MNT_UIDMAP, 0); > probe(id, STATMOUNT_FS_SUBTYPE, 0); > probe(id, STATMOUNT_MNT_UIDMAP, 1); > return 0; > } >=20 > Before, on 7.3.0-rc7-00252-g4e3259b08c54 (vfs.fixes) in a VM, the > same on a distro 7.1.13: >=20 > mask 0x2000: ret 0 errno 0, size 0x65, byte[512] 0x00 > mask 0x0100: ret 0 errno 0, size 0x65, byte[512] 0x00 > mask 0x2000 guard page: ret -1 errno 14 >=20 > After: >=20 > mask 0x2000: ret -1 errno 75, size 0xaaaaaaaa, byte[512] 0xaa > mask 0x0100: ret -1 errno 75, size 0xaaaaaaaa, byte[512] 0xaa > mask 0x2000 guard page: ret -1 errno 75 >=20 > Reported-by: shaos > Link: https://lore.kernel.org/r/20261020110747.959829-1-shawnq1@163.com > Fixes: e52e97f09fb6 ("statmount: let unset strings be empty") > Cc: stable@vger.kernel.org > Signed-off-by: Christian Brauner (Amutable) > --- > fs/namespace.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) >=20 > diff --git a/fs/namespace.c b/fs/namespace.c > index c30f063742f4..6c9c0cdbe890 100644 > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -6058,7 +6058,8 @@ static int prepare_kstatmount(struct kstatmount *ks= , struct mnt_id_req *kreq, > ks->bufsize =3D bufsize; > =20 > if (ks->mask & STATMOUNT_STRING_REQ) { > - if (bufsize =3D=3D sizeof(ks->sm)) > + /* Even the reserved empty string needs room past the fixed part. */ > + if (bufsize <=3D sizeof(ks->sm)) > return -EOVERFLOW; > =20 > ks->seq.buf =3D kvmalloc(seq_size, GFP_KERNEL_ACCOUNT); Reviewed-by: Jeff Layton