Re: [PATCH v4] ceph: bound encrypted snapshot suffix formatting

[Luis Henriques <luis@igalia.com>]

On Wed, Apr 22 2026, Ilya Dryomov wrote:

> On Fri, Apr 10, 2026 at 10:46 PM Viacheslav Dubeyko
> <Slava.Dubeyko@ibm.com> wrote:
>>
>> On Fri, 2026-04-10 at 20:40 +0000, Viacheslav Dubeyko wrote:
>> > On Thu, 2026-04-09 at 18:09 +0000, Viacheslav Dubeyko wrote:
>> > > On Thu, 2026-04-09 at 10:39 +0800, Pengpeng Hou wrote:
>> > > > ceph_encode_encrypted_dname() base64-encodes the encrypted snapshot
>> > > > name into the caller buffer and then, for long snapshot names, appends
>> > > > _<ino> with sprintf(p + elen, ...).
>> > > >
>> > > > Some callers only provide NAME_MAX bytes. For long snapshot names, a
>> > > > large inode suffix can push the final encoded name past NAME_MAX even
>> > > > though the encrypted prefix stayed within the documented 240-byte
>> > > > budget.
>> > > >
>> > > > Format the suffix into a small local buffer first and reject names
>> > > > whose suffix would exceed the caller's NAME_MAX output buffer.
>> > > >
>> > > > Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
>> > > > ---
>> > > > Changes since v3:
>> > > > - reject `elen > 240` explicitly instead of relying only on the earlier
>> > > >   `WARN_ON()`
>> > > > - rewrite the NAME_MAX bound check in terms of the final total length
>> > > >   instead of `NAME_MAX - prefix_len - elen`
>> > > >
>> > > >  fs/ceph/crypto.c | 31 +++++++++++++++++++++++++++++--
>> > > >  1 file changed, 29 insertions(+), 2 deletions(-)
>> > > >
>> > > > diff --git a/fs/ceph/crypto.c b/fs/ceph/crypto.c
>> > > > index f3de43ccb470..42e3fff34697 100644
>> > > > --- a/fs/ceph/crypto.c
>> > > > +++ b/fs/ceph/crypto.c
>> > > > @@ -15,6 +15,12 @@
>> > > >  #include "mds_client.h"
>> > > >  #include "crypto.h"
>> > > >
>> > > > +/*
>> > > > + * Reserve room for '_' + decimal 64-bit inode number + trailing NUL.
>> > > > + * ceph_encode_encrypted_dname() copies only the visible suffix bytes.
>> > > > + */
>> > > > +#define CEPH_ENCRYPTED_SNAP_INO_SUFFIX_MAX       sizeof("_18446744073709551615")
>> > > > +
>> > > >  static int ceph_crypt_get_context(struct inode *inode, void *ctx, size_t len)
>> > > >  {
>> > > >   struct ceph_inode_info *ci = ceph_inode(inode);
>> > > > @@ -209,6 +215,7 @@ int ceph_encode_encrypted_dname(struct inode *parent, char *buf, int elen)
>> > > >   struct inode *dir = parent;
>> > > >   char *p = buf;
>> > > >   u32 len;
>> > > > + int prefix_len = 0;
>> > > >   int name_len = elen;
>> > > >   int ret;
>> > > >   u8 *cryptbuf = NULL;
>> > > > @@ -219,6 +226,7 @@ int ceph_encode_encrypted_dname(struct inode *parent, char *buf, int elen)
>> > > >           if (IS_ERR(dir))
>> > > >                   return PTR_ERR(dir);
>> > > >           p++; /* skip initial '_' */
>> > > > +         prefix_len = 1;
>> > > >   }
>> > > >
>> > > >   if (!fscrypt_has_encryption_key(dir))
>> > > > @@ -271,8 +279,27 @@ int ceph_encode_encrypted_dname(struct inode *parent, char *buf, int elen)
>> > > >
>> > > >   /* To understand the 240 limit, see CEPH_NOHASH_NAME_MAX comments */
>> > > >   WARN_ON(elen > 240);
>> > > > - if (dir != parent) // leading _ is already there; append _<inum>
>> > > > -         elen += 1 + sprintf(p + elen, "_%ld", dir->i_ino);
>> > > > + if (elen > 240) {
>> > > > +         elen = -ENAMETOOLONG;
>> > > > +         goto out;
>> > > > + }
>> > > > +
>> > > > + if (dir != parent) {
>> > > > +         int total_len;
>> > > > +         /* leading '_' is already there; append _<inum> */
>> > > > +         char suffix[CEPH_ENCRYPTED_SNAP_INO_SUFFIX_MAX];
>> > > > +
>> > > > +         ret = snprintf(suffix, sizeof(suffix), "_%lu", dir->i_ino);
>> > > > +         total_len = prefix_len + elen + ret;
>> > > > +         if (total_len > NAME_MAX) {
>> > > > +                 elen = -ENAMETOOLONG;
>> > > > +                 goto out;
>> > > > +         }
>> > > > +
>> > > > +         memcpy(p + elen, suffix, ret);
>> > > > +         /* Include the leading '_' skipped by p. */
>> > > > +         elen = total_len;
>> > > > + }
>> > > >
>> > > >  out:
>> > > >   kfree(cryptbuf);
>> > >
>> > > Looks good.
>> > >
>> > > Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
>> > >
>> > > Let me run xfstests for the patch to double check that everything is OK. I'll
>> > > share the result ASAP.
>> > >
>> >
>> > The xfstests run was successful. I don't see any issues with the patch.
>> >
>> > Tested-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
>> >
>> >
>>
>> Applied on testing branch of CephFS kernel client git tree.
>
> Hi Pengpeng, Slava,
>
> This patch raised my attention because my understanding was that the
> entire CEPH_NOHASH_NAME_MAX + sha256() was put in place precisely to
> handle longer names nicely and make them fit into NAME_MAX-sized buffer.
> Simply rejecting longer names seemed to be in direct contradiction with
> that and yet the patch on its own was clearly merited given
>
>  * (240 bytes is the maximum size allowed for snapshot names to take into
>  *  account the format: '_<SNAPSHOT-NAME>_<INODE-NUMBER>'.)
>
> comment on CEPH_NOHASH_NAME_MAX definition.
>
> I dug a bit deeper and started a discussion in [1].  The preliminary
> conclusion is that the 240 bytes assumption was a mistake -- somehow
> the minimum number of characters needed for <inum> ended up being used
> instead of the maximum.  CEPH_NOHASH_NAME_MAX value is likely incorrect
> and should have been smaller -- something along the lines of 174 -
> SHA256_DIGEST_SIZE instead of 180 - SHA256_DIGEST_SIZE.

FWIW I _think_ Ilya may be correct, and there's actually an issue when
these constants were defined.  I spent quite some time last evening trying
to dig into the details on why the inode length was assumed to be 13.
Apparently, it was just a mistake that no one spotted at the time :-(

Unfortunately, my bandwidth to look into this is quite limited.  But I
believe it should be easy to verify that this is indeed a bug by simply
creating snapshots on an encrypted directory that has an inode number
bigger than 2^40 and then checking the large snapshot name in the '.snap'
directory of a subdirectory.

Cheers,
-- 
Luís

> I kept this patch in the testing branch but not including it for 7.1
> pending further investigation.
>
> [1] https://github.com/ceph/ceph/pull/45312
>
> Thanks,
>
>                 Ilya