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: Hanna Reitz <hreitz@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 6511a6acb4..b43afdfbd3 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -3123,6 +3123,7 @@ 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 changed_cwd = false;
 
     if (block_xattr(lo, in_name)) {
         fuse_reply_err(req, EOPNOTSUPP);
@@ -3158,6 +3159,24 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
              ", name=%s value=%s size=%zd)\n", ino, name, value, size);
 
     sprintf(procname, "%i", inode->fd);
+    /*
+     * 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.
+     */
+    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;
+        }
+    } else {
+        /* fchdir should not fail here */
+        FCHDIR_NOFAIL(lo->proc_self_fd);
+        /* Set flag so the clean-up path will chdir back */
+        changed_cwd = true;
+    }
+
     /*
      * If we are setting posix access acl and if SGID needs to be
      * cleared, then switch to caller's gid and drop CAP_FSETID
@@ -3178,20 +3197,12 @@ 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 (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)
@@ -3201,6 +3212,11 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
     }
 
 out:
+    if (changed_cwd) {
+        /* Change CWD back, fchdir should not fail here */
+        FCHDIR_NOFAIL(lo->root.fd);
+    }
+
     if (fd >= 0) {
         close(fd);
     }
-- 
2.31.1

On Thu, Sep 16, 2021 at 10:40:35AM +0200, Hanna 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: Hanna Reitz <hreitz@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 6511a6acb4..b43afdfbd3 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -3123,6 +3123,7 @@ 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 changed_cwd = false;
>  
>      if (block_xattr(lo, in_name)) {
>          fuse_reply_err(req, EOPNOTSUPP);
> @@ -3158,6 +3159,24 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
>               ", name=%s value=%s size=%zd)\n", ino, name, value, size);
>  
>      sprintf(procname, "%i", inode->fd);
> +    /*
> +     * 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.
> +     */
> +    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;
> +        }
> +    } else {
> +        /* fchdir should not fail here */
> +        FCHDIR_NOFAIL(lo->proc_self_fd);
> +        /* Set flag so the clean-up path will chdir back */
> +        changed_cwd = true;

Is there a need to move FCHDIR_NOFAIL() call earlier too? I am assuming
this will not be impacted by file handle stuff. So we probably could
leave it in place. Easier to read.

Vivek

> +    }
> +
>      /*
>       * If we are setting posix access acl and if SGID needs to be
>       * cleared, then switch to caller's gid and drop CAP_FSETID
> @@ -3178,20 +3197,12 @@ 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 (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)
> @@ -3201,6 +3212,11 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
>      }
>  
>  out:
> +    if (changed_cwd) {
> +        /* Change CWD back, fchdir should not fail here */
> +        FCHDIR_NOFAIL(lo->root.fd);
> +    }
> +
>      if (fd >= 0) {
>          close(fd);
>      }
> -- 
> 2.31.1
> 

On 18.10.21 19:20, Vivek Goyal wrote:
> On Thu, Sep 16, 2021 at 10:40:35AM +0200, Hanna 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: Hanna Reitz <hreitz@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 6511a6acb4..b43afdfbd3 100644
>> --- a/tools/virtiofsd/passthrough_ll.c
>> +++ b/tools/virtiofsd/passthrough_ll.c
>> @@ -3123,6 +3123,7 @@ 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 changed_cwd = false;
>>   
>>       if (block_xattr(lo, in_name)) {
>>           fuse_reply_err(req, EOPNOTSUPP);
>> @@ -3158,6 +3159,24 @@ static void lo_setxattr(fuse_req_t req, fuse_ino_t ino, const char *in_name,
>>                ", name=%s value=%s size=%zd)\n", ino, name, value, size);
>>   
>>       sprintf(procname, "%i", inode->fd);
>> +    /*
>> +     * 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.
>> +     */
>> +    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;
>> +        }
>> +    } else {
>> +        /* fchdir should not fail here */
>> +        FCHDIR_NOFAIL(lo->proc_self_fd);
>> +        /* Set flag so the clean-up path will chdir back */
>> +        changed_cwd = true;
> Is there a need to move FCHDIR_NOFAIL() call earlier too? I am assuming
> this will not be impacted by file handle stuff. So we probably could
> leave it in place. Easier to read.

I wanted to limit the region where the creds are dropped to an absolute 
minimum, i.e. just around (f)setxattr().  I prefer this in general, not 
just because it breaks opening file handles, and so I wanted to pull out 
not just the openat(), but the fchdir() as well.

Hanna