Message ID | 20210304103858.38400-2-dgilbert@redhat.com |
---|---|
State | New |
Headers | show |
Series | [PULL,1/1] virtiofs: drop remapped security.capability xattr as needed | expand |
Oops, forgot to copy in qemu-stable; since xattrmap is in 5.2.0 it probably should go into stable. (Taking 1e08f16 'virtiofsd: Save error code early at the failure callsite' makes it easier to backport) Dave * Dr. David Alan Gilbert (git) (dgilbert@redhat.com) wrote: > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > On Linux, the 'security.capability' xattr holds a set of > capabilities that can change when an executable is run, giving > a limited form of privilege escalation to those programs that > the writer of the file deemed worthy. > > Any write causes the 'security.capability' xattr to be dropped, > stopping anyone from gaining privilege by modifying a blessed > file. > > Fuse relies on the daemon to do this dropping, and in turn the > daemon relies on the host kernel to drop the xattr for it. However, > with the addition of -o xattrmap, the xattr that the guest > stores its capabilities in is now not the same as the one that > the host kernel automatically clears. > > Where the mapping changes 'security.capability', explicitly clear > the remapped name to preserve the same behaviour. > > This bug is assigned CVE-2021-20263. > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > Reviewed-by: Vivek Goyal <vgoyal@redhat.com> > --- > docs/tools/virtiofsd.rst | 4 ++ > tools/virtiofsd/passthrough_ll.c | 77 +++++++++++++++++++++++++++++++- > 2 files changed, 80 insertions(+), 1 deletion(-) > > diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst > index 866b7db3ee..00554c75bd 100644 > --- a/docs/tools/virtiofsd.rst > +++ b/docs/tools/virtiofsd.rst > @@ -228,6 +228,10 @@ The 'map' type adds a number of separate rules to add **prepend** as a prefix > to the matched **key** (or all attributes if **key** is empty). > There may be at most one 'map' rule and it must be the last rule in the set. > > +Note: When the 'security.capability' xattr is remapped, the daemon has to do > +extra work to remove it during many operations, which the host kernel normally > +does itself. > + > xattr-mapping Examples > ---------------------- > > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c > index 58d24c0010..fc7e1b1e8e 100644 > --- a/tools/virtiofsd/passthrough_ll.c > +++ b/tools/virtiofsd/passthrough_ll.c > @@ -148,6 +148,7 @@ struct lo_data { > int posix_lock; > int xattr; > char *xattrmap; > + char *xattr_security_capability; > char *source; > char *modcaps; > double timeout; > @@ -217,6 +218,8 @@ static __thread bool cap_loaded = 0; > > static struct lo_inode *lo_find(struct lo_data *lo, struct stat *st, > uint64_t mnt_id); > +static int xattr_map_client(const struct lo_data *lo, const char *client_name, > + char **out_name); > > static int is_dot_or_dotdot(const char *name) > { > @@ -356,6 +359,37 @@ out: > return ret; > } > > +/* > + * The host kernel normally drops security.capability xattr's on > + * any write, however if we're remapping xattr names we need to drop > + * whatever the clients security.capability is actually stored as. > + */ > +static int drop_security_capability(const struct lo_data *lo, int fd) > +{ > + if (!lo->xattr_security_capability) { > + /* We didn't remap the name, let the host kernel do it */ > + return 0; > + } > + if (!fremovexattr(fd, lo->xattr_security_capability)) { > + /* All good */ > + return 0; > + } > + > + switch (errno) { > + case ENODATA: > + /* Attribute didn't exist, that's fine */ > + return 0; > + > + case ENOTSUP: > + /* FS didn't support attribute anyway, also fine */ > + return 0; > + > + default: > + /* Hmm other error */ > + return errno; > + } > +} > + > static void lo_map_init(struct lo_map *map) > { > map->elems = NULL; > @@ -737,6 +771,11 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr, > uid_t uid = (valid & FUSE_SET_ATTR_UID) ? attr->st_uid : (uid_t)-1; > gid_t gid = (valid & FUSE_SET_ATTR_GID) ? attr->st_gid : (gid_t)-1; > > + saverr = drop_security_capability(lo, ifd); > + if (saverr) { > + goto out_err; > + } > + > res = fchownat(ifd, "", uid, gid, AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW); > if (res == -1) { > saverr = errno; > @@ -759,6 +798,14 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr, > } > } > > + saverr = drop_security_capability(lo, truncfd); > + if (saverr) { > + if (!fi) { > + close(truncfd); > + } > + goto out_err; > + } > + > if (kill_suidgid) { > res = drop_effective_cap("FSETID", &cap_fsetid_dropped); > if (res != 0) { > @@ -1784,6 +1831,13 @@ static int lo_do_open(struct lo_data *lo, struct lo_inode *inode, > if (fd < 0) { > return -fd; > } > + if (fi->flags & (O_TRUNC)) { > + int err = drop_security_capability(lo, fd); > + if (err) { > + close(fd); > + return err; > + } > + } > } > > pthread_mutex_lock(&lo->mutex); > @@ -2191,6 +2245,12 @@ static void lo_write_buf(fuse_req_t req, fuse_ino_t ino, > "lo_write_buf(ino=%" PRIu64 ", size=%zd, off=%lu kill_priv=%d)\n", > ino, out_buf.buf[0].size, (unsigned long)off, fi->kill_priv); > > + res = drop_security_capability(lo_data(req), out_buf.buf[0].fd); > + if (res) { > + fuse_reply_err(req, res); > + return; > + } > + > /* > * If kill_priv is set, drop CAP_FSETID which should lead to kernel > * clearing setuid/setgid on file. Note, for WRITE, we need to do > @@ -2432,6 +2492,7 @@ static void parse_xattrmap(struct lo_data *lo) > { > const char *map = lo->xattrmap; > const char *tmp; > + int ret; > > lo->xattr_map_nentries = 0; > while (*map) { > @@ -2462,7 +2523,7 @@ static void parse_xattrmap(struct lo_data *lo) > * the last entry. > */ > parse_xattrmap_map(lo, map, sep); > - return; > + break; > } else { > fuse_log(FUSE_LOG_ERR, > "%s: Unexpected type;" > @@ -2531,6 +2592,19 @@ static void parse_xattrmap(struct lo_data *lo) > fuse_log(FUSE_LOG_ERR, "Empty xattr map\n"); > exit(1); > } > + > + ret = xattr_map_client(lo, "security.capability", > + &lo->xattr_security_capability); > + if (ret) { > + fuse_log(FUSE_LOG_ERR, "Failed to map security.capability: %s\n", > + strerror(ret)); > + exit(1); > + } > + if (!strcmp(lo->xattr_security_capability, "security.capability")) { > + /* 1-1 mapping, don't need to do anything */ > + free(lo->xattr_security_capability); > + lo->xattr_security_capability = NULL; > + } > } > > /* > @@ -3588,6 +3662,7 @@ static void fuse_lo_data_cleanup(struct lo_data *lo) > > free(lo->xattrmap); > free_xattrmap(lo); > + free(lo->xattr_security_capability); > free(lo->source); > } > > -- > 2.29.2 > >
diff --git a/docs/tools/virtiofsd.rst b/docs/tools/virtiofsd.rst index 866b7db3ee..00554c75bd 100644 --- a/docs/tools/virtiofsd.rst +++ b/docs/tools/virtiofsd.rst @@ -228,6 +228,10 @@ The 'map' type adds a number of separate rules to add **prepend** as a prefix to the matched **key** (or all attributes if **key** is empty). There may be at most one 'map' rule and it must be the last rule in the set. +Note: When the 'security.capability' xattr is remapped, the daemon has to do +extra work to remove it during many operations, which the host kernel normally +does itself. + xattr-mapping Examples ---------------------- diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c index 58d24c0010..fc7e1b1e8e 100644 --- a/tools/virtiofsd/passthrough_ll.c +++ b/tools/virtiofsd/passthrough_ll.c @@ -148,6 +148,7 @@ struct lo_data { int posix_lock; int xattr; char *xattrmap; + char *xattr_security_capability; char *source; char *modcaps; double timeout; @@ -217,6 +218,8 @@ static __thread bool cap_loaded = 0; static struct lo_inode *lo_find(struct lo_data *lo, struct stat *st, uint64_t mnt_id); +static int xattr_map_client(const struct lo_data *lo, const char *client_name, + char **out_name); static int is_dot_or_dotdot(const char *name) { @@ -356,6 +359,37 @@ out: return ret; } +/* + * The host kernel normally drops security.capability xattr's on + * any write, however if we're remapping xattr names we need to drop + * whatever the clients security.capability is actually stored as. + */ +static int drop_security_capability(const struct lo_data *lo, int fd) +{ + if (!lo->xattr_security_capability) { + /* We didn't remap the name, let the host kernel do it */ + return 0; + } + if (!fremovexattr(fd, lo->xattr_security_capability)) { + /* All good */ + return 0; + } + + switch (errno) { + case ENODATA: + /* Attribute didn't exist, that's fine */ + return 0; + + case ENOTSUP: + /* FS didn't support attribute anyway, also fine */ + return 0; + + default: + /* Hmm other error */ + return errno; + } +} + static void lo_map_init(struct lo_map *map) { map->elems = NULL; @@ -737,6 +771,11 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr, uid_t uid = (valid & FUSE_SET_ATTR_UID) ? attr->st_uid : (uid_t)-1; gid_t gid = (valid & FUSE_SET_ATTR_GID) ? attr->st_gid : (gid_t)-1; + saverr = drop_security_capability(lo, ifd); + if (saverr) { + goto out_err; + } + res = fchownat(ifd, "", uid, gid, AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW); if (res == -1) { saverr = errno; @@ -759,6 +798,14 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr, } } + saverr = drop_security_capability(lo, truncfd); + if (saverr) { + if (!fi) { + close(truncfd); + } + goto out_err; + } + if (kill_suidgid) { res = drop_effective_cap("FSETID", &cap_fsetid_dropped); if (res != 0) { @@ -1784,6 +1831,13 @@ static int lo_do_open(struct lo_data *lo, struct lo_inode *inode, if (fd < 0) { return -fd; } + if (fi->flags & (O_TRUNC)) { + int err = drop_security_capability(lo, fd); + if (err) { + close(fd); + return err; + } + } } pthread_mutex_lock(&lo->mutex); @@ -2191,6 +2245,12 @@ static void lo_write_buf(fuse_req_t req, fuse_ino_t ino, "lo_write_buf(ino=%" PRIu64 ", size=%zd, off=%lu kill_priv=%d)\n", ino, out_buf.buf[0].size, (unsigned long)off, fi->kill_priv); + res = drop_security_capability(lo_data(req), out_buf.buf[0].fd); + if (res) { + fuse_reply_err(req, res); + return; + } + /* * If kill_priv is set, drop CAP_FSETID which should lead to kernel * clearing setuid/setgid on file. Note, for WRITE, we need to do @@ -2432,6 +2492,7 @@ static void parse_xattrmap(struct lo_data *lo) { const char *map = lo->xattrmap; const char *tmp; + int ret; lo->xattr_map_nentries = 0; while (*map) { @@ -2462,7 +2523,7 @@ static void parse_xattrmap(struct lo_data *lo) * the last entry. */ parse_xattrmap_map(lo, map, sep); - return; + break; } else { fuse_log(FUSE_LOG_ERR, "%s: Unexpected type;" @@ -2531,6 +2592,19 @@ static void parse_xattrmap(struct lo_data *lo) fuse_log(FUSE_LOG_ERR, "Empty xattr map\n"); exit(1); } + + ret = xattr_map_client(lo, "security.capability", + &lo->xattr_security_capability); + if (ret) { + fuse_log(FUSE_LOG_ERR, "Failed to map security.capability: %s\n", + strerror(ret)); + exit(1); + } + if (!strcmp(lo->xattr_security_capability, "security.capability")) { + /* 1-1 mapping, don't need to do anything */ + free(lo->xattr_security_capability); + lo->xattr_security_capability = NULL; + } } /* @@ -3588,6 +3662,7 @@ static void fuse_lo_data_cleanup(struct lo_data *lo) free(lo->xattrmap); free_xattrmap(lo); + free(lo->xattr_security_capability); free(lo->source); }