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

[Dr. David Alan Gilbert]

* Dr. David Alan Gilbert (address@hidden) wrote:
> * 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.

Hmm hang on; this is messy.  x86-64 only seems to have setresuid
where as some architectures have both;  If I'm reading this right, all
64 bit machines have setresuid/gid calling the code that takes the
32bit ID; some have compat entries for 32bit syscalls.

I think it's probably more correct to call setresuid here; except
for 32 bit platforms - but how do we tell?

Dave

> 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
> > -- 
> > |: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange 
> > :|
> > |: https://libvirt.org         -o-            https://fstop138.berrange.com 
> > :|
> > |: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange 
> > :|
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK