[Virtio-fs] [PATCH v3 01/10] virtiofsd: Limit setxattr()'s creds-dropped region
Max Reitz
mreitz at redhat.com
Mon Aug 9 10:30:40 UTC 2021
On 06.08.21 16:16, Vivek Goyal wrote:
> On Fri, Jul 30, 2021 at 05:01:25PM +0200, Max Reitz wrote:
>> We only need to drop/switch our credentials for the (f)setxattr() call
>> alone, not for the openat() or fchdir() around it.
>>
>> (Right now, this may not be that big of a problem, but with inodes being
>> identified by file handles instead of an O_PATH fd, we will need
>> open_by_handle_at() calls here, which is really fickle when it comes to
>> credentials being dropped.)
>>
>> Signed-off-by: Max Reitz <mreitz at redhat.com>
>> ---
>> tools/virtiofsd/passthrough_ll.c | 34 +++++++++++++++++++++++---------
>> 1 file changed, 25 insertions(+), 9 deletions(-)
>>
>> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
>> index 38b2af8599..1f27eeabc5 100644
>> --- a/tools/virtiofsd/passthrough_ll.c
>> +++ b/tools/virtiofsd/passthrough_ll.c
>> @@ -3121,6 +3121,7 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
>> bool switched_creds = false;
>> bool cap_fsetid_dropped = false;
>> struct lo_cred old = {};
>> + bool open_inode;
>>
>> if (block_xattr(lo, in_name)) {
>> fuse_reply_err(req, EOPNOTSUPP);
>> @@ -3155,7 +3156,24 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
>> fuse_log(FUSE_LOG_DEBUG, "lo_setxattr(ino=%" PRIu64
>> ", name=%s value=%s size=%zd)\n", ino, name, value, size);
>>
>> + /*
>> + * We can only open regular files or directories. If the inode is
>> + * something else, we have to enter /proc/self/fd and use
>> + * setxattr() on the link's filename there.
>> + */
>> + open_inode = S_ISREG(inode->filetype) || S_ISDIR(inode->filetype);
>> sprintf(procname, "%i", inode->fd);
>> + if (open_inode) {
>> + fd = openat(lo->proc_self_fd, procname, O_RDONLY);
>> + if (fd < 0) {
>> + saverr = errno;
>> + goto out;
>> + }
>> + } else {
>> + /* fchdir should not fail here */
>> + FCHDIR_NOFAIL(lo->proc_self_fd);
>> + }
>> +
>> /*
>> * If we are setting posix access acl and if SGID needs to be
>> * cleared, then switch to caller's gid and drop CAP_FSETID
>> @@ -3176,20 +3194,13 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
>> }
>> switched_creds = true;
>> }
>> - if (S_ISREG(inode->filetype) || S_ISDIR(inode->filetype)) {
>> - fd = openat(lo->proc_self_fd, procname, O_RDONLY);
>> - if (fd < 0) {
>> - saverr = errno;
>> - goto out;
>> - }
>> + if (open_inode) {
>> + assert(fd >= 0);
>> ret = fsetxattr(fd, name, value, size, flags);
>> saverr = ret == -1 ? errno : 0;
>> } else {
>> - /* fchdir should not fail here */
>> - FCHDIR_NOFAIL(lo->proc_self_fd);
>> ret = setxattr(procname, name, value, size, flags);
>> saverr = ret == -1 ? errno : 0;
>> - FCHDIR_NOFAIL(lo->root.fd);
>> }
>> if (switched_creds) {
>> if (cap_fsetid_dropped)
>> @@ -3198,6 +3209,11 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
>> lo_restore_cred(&old, false);
>> }
>>
>> + if (!open_inode) {
>> + /* Change CWD back, fchdir should not fail here */
>> + FCHDIR_NOFAIL(lo->root.fd);
>> + }
>> +
> This FCHDIR_NOFAIL() will also need to be called if lo_drop_cap_change_cred()
> fails.
>
> ret = lo_drop_cap_change_cred(req, &old, false, "FSETID",
> &cap_fsetid_dropped);
> if (ret) {
> saverr = ret;
> goto out;
> }
Oh, right, thanks!
Max
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