| From aaa5f8e00c2e85a893b972f1e243fb14c26b70dc Mon Sep 17 00:00:00 2001 |
| From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> |
| Date: Wed, 24 Feb 2021 19:56:25 +0000 |
| Subject: [PATCH 2/2] virtiofs: drop remapped security.capability xattr as |
| needed |
| |
| 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> |
| |
| Upstream-Status: Backport [e586edcb410543768ef009eaa22a2d9dd4a53846] |
| CVE: CVE-2021-20263 |
| |
| Signed-off-by: Sakib Sajal <sakib.sajal@windriver.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 866b7db3e..00554c75b 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 03c5e0d13..c9197da86 100644 |
| --- a/tools/virtiofsd/passthrough_ll.c |
| +++ b/tools/virtiofsd/passthrough_ll.c |
| @@ -160,6 +160,7 @@ struct lo_data { |
| int posix_lock; |
| int xattr; |
| char *xattrmap; |
| + char *xattr_security_capability; |
| char *source; |
| char *modcaps; |
| double timeout; |
| @@ -226,6 +227,8 @@ 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) |
| { |
| @@ -365,6 +368,37 @@ 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; |
| @@ -717,6 +751,11 @@ 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) { |
| goto out_err; |
| @@ -735,6 +774,14 @@ 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; |
| + } |
| + |
| res = ftruncate(truncfd, attr->st_size); |
| if (!fi) { |
| saverr = errno; |
| @@ -1726,6 +1773,13 @@ 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); |
| @@ -2114,6 +2168,12 @@ static void lo_write_buf(fuse_req_t req, fuse_ino_t ino, |
| "lo_write_buf(ino=%" PRIu64 ", size=%zd, off=%lu)\n", ino, |
| out_buf.buf[0].size, (unsigned long)off); |
| |
| + 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. |
| @@ -2353,6 +2413,7 @@ 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) { |
| @@ -2383,7 +2444,7 @@ 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;" |
| @@ -2452,6 +2513,19 @@ 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; |
| + } |
| } |
| |
| /* |
| @@ -3480,6 +3554,7 @@ 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 |
| |