On Wed, Nov 18, 2020 at 5:23 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote: > > When SELinux security options are passed to btrfs via fsconfig(2) rather > than via mount(2), the operation aborts with an error. What happens is > roughly this sequence: > > 1. vfs_parse_fs_param() eats away the LSM options and parses them into > fc->security. > 2. legacy_get_tree() finds nothing in ctx->legacy_data, passes this > nothing to btrfs. > [here btrfs calls another layer of vfs_kern_mount(), but let's ignore > that for simplicity] > 3. btrfs calls security_sb_set_mnt_opts() with empty options. > 4. vfs_get_tree() then calls its own security_sb_set_mnt_opts() with the > options stashed in fc->security. > 5. SELinux doesn't like that different options were used for the same > superblock and returns -EINVAL. > > In the case of mount(2), the options are parsed by > legacy_parse_monolithic(), which skips the eating away of security > opts because of the FS_BINARY_MOUNTDATA flag, so they are passed to the > FS via ctx->legacy_data. The second call to security_sb_set_mnt_opts() > (from vfs_get_tree()) now passes empty opts, but the non-empty -> empty > sequence is allowed by SELinux for the FS_BINARY_MOUNTDATA case. > > It is a total mess, but the only sane fix for now seems to be to skip > processing the security opts in vfs_parse_fs_param() if the fc has > legacy opts set AND the fs specfies the FS_BINARY_MOUNTDATA flag. This > combination currently matches only btrfs and coda. For btrfs this fixes > the fsconfig(2) behavior, and for coda it makes setting security opts > via fsconfig(2) fail the same way as it would with mount(2) (because > FS_BINARY_MOUNTDATA filesystems are expected to call the mount opts LSM > hooks themselves, but coda never cared enough to do that). I believe > that is an acceptable state until both filesystems (or at least btrfs) > are converted to the new mount API (at which point btrfs won't need to > pretend it takes binary mount data any more and also won't need to call > the LSM hooks itself, assuming it will pass the fc->security information > properly). > > Note that we can't skip LSM opts handling in vfs_parse_fs_param() solely > based on FS_BINARY_MOUNTDATA because that would break NFS. > > See here for the original report and reproducer: > https://lore.kernel.org/selinux/c02674c970fa292610402aa866c4068772d9ad4e.camel@xxxxxxxxxxxxxx/ > > Reported-by: Richard Haines <richard_c_haines@xxxxxxxxxxxxxx> > Fixes: 3e1aeb00e6d1 ("vfs: Implement a filesystem superblock creation/configuration context") > Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx> > --- > fs/fs_context.c | 28 ++++++++++++++++++++++------ > 1 file changed, 22 insertions(+), 6 deletions(-) What do the VFS folks think about this patch? It has been sitting for a couple of months now without any real comment and it would be nice to get some initial feedback on this as it fixes a real problem. > diff --git a/fs/fs_context.c b/fs/fs_context.c > index 2834d1afa6e80..cfc5ee2e381ef 100644 > --- a/fs/fs_context.c > +++ b/fs/fs_context.c > @@ -106,12 +106,28 @@ int vfs_parse_fs_param(struct fs_context *fc, struct fs_parameter *param) > if (ret != -ENOPARAM) > return ret; > > - ret = security_fs_context_parse_param(fc, param); > - if (ret != -ENOPARAM) > - /* Param belongs to the LSM or is disallowed by the LSM; so > - * don't pass to the FS. > - */ > - return ret; > + /* > + * In the legacy+binary mode, skip the security_fs_context_parse_param() > + * call and let the legacy handler process also the security options. > + * It will format them into the monolithic string, where the FS can > + * process them (with FS_BINARY_MOUNTDATA it is expected to do it). > + * > + * Currently, this matches only btrfs and coda. Coda is broken with > + * fsconfig(2) anyway, because it does actually take binary data. Btrfs > + * only *pretends* to take binary data to work around the SELinux's > + * no-remount-with-different-options check, so this allows it to work > + * with fsconfig(2) properly. > + * > + * Once btrfs is ported to the new mount API, this hack can be reverted. > + */ > + if (fc->ops != &legacy_fs_context_ops || !(fc->fs_type->fs_flags & FS_BINARY_MOUNTDATA)) { > + ret = security_fs_context_parse_param(fc, param); > + if (ret != -ENOPARAM) > + /* Param belongs to the LSM or is disallowed by the LSM; > + * so don't pass to the FS. > + */ > + return ret; > + } > > if (fc->ops->parse_param) { > ret = fc->ops->parse_param(fc, param); > -- > 2.26.2 -- paul moore www.paul-moore.com
