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

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (address@hidden) wrote:
> 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]

OK, updated.

Dave

> > +    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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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK