Re: [PATCH 032/104] virtiofsd: passthrough_ll: create new files in caller's context

[Daniel P . Berrangé]

On Thu, Dec 12, 2019 at 04:37:52PM +0000, Dr. David Alan Gilbert (git) wrote:
> From: Vivek Goyal <address@hidden>
> 
> We need to create files in the caller's context. Otherwise after
> creating a file, the caller might not be able to do file operations on
> that file.
> 
> Changed effective uid/gid to caller's uid/gid, create file and then
> switch back to uid/gid 0.
> 
> Use syscall(setresuid, ...) otherwise glibc does some magic to change EUID
> in all threads, which is not what we want.
> 
> Signed-off-by: Vivek Goyal <address@hidden>
> Signed-off-by: Miklos Szeredi <address@hidden>
> ---
>  tools/virtiofsd/passthrough_ll.c | 79 ++++++++++++++++++++++++++++++--
>  1 file changed, 74 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/virtiofsd/passthrough_ll.c 
> b/tools/virtiofsd/passthrough_ll.c
> index 68bacb6fc5..0188cd9ad6 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c


> +static int lo_change_cred(fuse_req_t req, struct lo_cred *old)
> +{
> +    int res;
> +
> +    old->euid = geteuid();
> +    old->egid = getegid();
> +
> +    res = syscall(SYS_setresgid, -1, fuse_req_ctx(req)->gid, -1);

Do we need to be using  SYS_setres[u,g]id32 instead...

[quote setresgid(2)]
       The original Linux setresuid() and setresgid() system  calls
       supported  only  16-bit  user  and group IDs.  Subsequently,
       Linux 2.4 added setresuid32() and setresgid32(),  supporting
       32-bit  IDs.   The glibc setresuid() and setresgid() wrapper
       functions transparently deal with the variations across ker‐
       nel versions.
[/quote]

> +    if (res == -1) {
> +        return errno;
> +    }
> +
> +    res = syscall(SYS_setresuid, -1, fuse_req_ctx(req)->uid, -1);
> +    if (res == -1) {
> +        int errno_save = errno;
> +
> +        syscall(SYS_setresgid, -1, old->egid, -1);
> +        return errno_save;
> +    }
> +
> +    return 0;
> +}
> +
> +/* Regain Privileges */
> +static void lo_restore_cred(struct lo_cred *old)
> +{
> +    int res;
> +
> +    res = syscall(SYS_setresuid, -1, old->euid, -1);
> +    if (res == -1) {
> +        fuse_log(FUSE_LOG_ERR, "seteuid(%u): %m\n", old->euid);
> +        exit(1);
> +    }
> +
> +    res = syscall(SYS_setresgid, -1, old->egid, -1);
> +    if (res == -1) {
> +        fuse_log(FUSE_LOG_ERR, "setegid(%u): %m\n", old->egid);
> +        exit(1);
> +    }
> +}
> +
>  static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t parent,
>                               const char *name, mode_t mode, dev_t rdev,
>                               const char *link)
> @@ -391,12 +443,21 @@ static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t 
> parent,
>      int saverr;
>      struct lo_inode *dir = lo_inode(req, parent);
>      struct fuse_entry_param e;
> +    struct lo_cred old = {};
>  
>      saverr = ENOMEM;
>  
> +    saverr = lo_change_cred(req, &old);
> +    if (saverr) {
> +        goto out;
> +    }
> +
>      res = mknod_wrapper(dir->fd, name, link, mode, rdev);
>  
>      saverr = errno;
> +
> +    lo_restore_cred(&old);
> +
>      if (res == -1) {
>          goto out;
>      }
> @@ -794,26 +855,34 @@ static void lo_create(fuse_req_t req, fuse_ino_t 
> parent, const char *name,
>      struct lo_data *lo = lo_data(req);
>      struct fuse_entry_param e;
>      int err;
> +    struct lo_cred old = {};
>  
>      if (lo_debug(req)) {
>          fuse_log(FUSE_LOG_DEBUG, "lo_create(parent=%" PRIu64 ", name=%s)\n",
>                   parent, name);
>      }
>  
> +    err = lo_change_cred(req, &old);
> +    if (err) {
> +        goto out;
> +    }
> +
>      fd = openat(lo_fd(req, parent), name, (fi->flags | O_CREAT) & 
> ~O_NOFOLLOW,
>                  mode);
> -    if (fd == -1) {
> -        return (void)fuse_reply_err(req, errno);
> -    }
> +    err = fd == -1 ? errno : 0;
> +    lo_restore_cred(&old);
>  
> -    fi->fh = fd;
> +    if (!err) {
> +        fi->fh = fd;
> +        err = lo_do_lookup(req, parent, name, &e);
> +    }
>      if (lo->cache == CACHE_NEVER) {
>          fi->direct_io = 1;
>      } else if (lo->cache == CACHE_ALWAYS) {
>          fi->keep_cache = 1;
>      }
>  
> -    err = lo_do_lookup(req, parent, name, &e);
> +out:
>      if (err) {
>          fuse_reply_err(req, err);
>      } else {
> -- 
> 2.23.0
> 
> 

Regards,
Daniel
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