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Date: Mon, 16 Aug 2021 16:06:57 +0200
From: Jan Kara <jack@...e.cz>
To: Amir Goldstein <amir73il@...il.com>
Cc: Gabriel Krisman Bertazi <krisman@...labora.com>,
Jan Kara <jack@...e.com>,
Linux API <linux-api@...r.kernel.org>,
Ext4 <linux-ext4@...r.kernel.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
Khazhismel Kumykov <khazhy@...gle.com>,
David Howells <dhowells@...hat.com>,
Dave Chinner <david@...morbit.com>,
Theodore Tso <tytso@....edu>,
"Darrick J. Wong" <djwong@...nel.org>,
Matthew Bobrowski <repnop@...gle.com>, kernel@...labora.com
Subject: Re: [PATCH v6 12/21] fanotify: Encode invalid file handle when no
inode is provided
On Fri 13-08-21 11:27:48, Amir Goldstein wrote:
> On Fri, Aug 13, 2021 at 12:41 AM Gabriel Krisman Bertazi
> <krisman@...labora.com> wrote:
> >
> > Instead of failing, encode an invalid file handle in fanotify_encode_fh
> > if no inode is provided. This bogus file handle will be reported by
> > FAN_FS_ERROR for non-inode errors.
> >
> > When being reported to userspace, the length information is actually
> > reset and the handle cleaned up, such that userspace don't have the
> > visibility of the internal kernel representation of this null handle.
> >
> > Also adjust the single caller that might rely on failure after passing
> > an empty inode.
> >
> > Suggested-by: Amir Goldstein <amir73il@...il.com>
> > Signed-off-by: Gabriel Krisman Bertazi <krisman@...labora.com>
> >
> > ---
> > Changes since v5:
> > - Preserve flags initialization (jan)
> > - Add BUILD_BUG_ON (amir)
> > - Require minimum of FANOTIFY_NULL_FH_LEN for fh_len(amir)
> > - Improve comment to explain the null FH length (jan)
> > - Simplify logic
> > ---
> > fs/notify/fanotify/fanotify.c | 27 ++++++++++++++++++-----
> > fs/notify/fanotify/fanotify_user.c | 35 +++++++++++++++++-------------
> > 2 files changed, 41 insertions(+), 21 deletions(-)
> >
> > diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
> > index 50fce4fec0d6..2b1ab031fbe5 100644
> > --- a/fs/notify/fanotify/fanotify.c
> > +++ b/fs/notify/fanotify/fanotify.c
> > @@ -334,6 +334,8 @@ static u32 fanotify_group_event_mask(struct fsnotify_group *group,
> > return test_mask & user_mask;
> > }
> >
> > +#define FANOTIFY_NULL_FH_LEN 4
> > +
> > /*
> > * Check size needed to encode fanotify_fh.
> > *
> > @@ -345,7 +347,7 @@ static int fanotify_encode_fh_len(struct inode *inode)
> > int dwords = 0;
> >
> > if (!inode)
> > - return 0;
> > + return FANOTIFY_NULL_FH_LEN;
> >
> > exportfs_encode_inode_fh(inode, NULL, &dwords, NULL);
> >
> > @@ -367,11 +369,23 @@ static int fanotify_encode_fh(struct fanotify_fh *fh, struct inode *inode,
> > void *buf = fh->buf;
> > int err;
> >
> > - fh->type = FILEID_ROOT;
> > - fh->len = 0;
> > + BUILD_BUG_ON(FANOTIFY_NULL_FH_LEN < 4 ||
> > + FANOTIFY_NULL_FH_LEN > FANOTIFY_INLINE_FH_LEN);
> > +
> > fh->flags = 0;
> > - if (!inode)
> > - return 0;
> > +
> > + if (!inode) {
> > + /*
> > + * Invalid FHs are used on FAN_FS_ERROR for errors not
> > + * linked to any inode. The f_handle won't be reported
> > + * back to userspace. The extra bytes are cleared prior
> > + * to reporting.
> > + */
> > + type = FILEID_INVALID;
> > + fh_len = FANOTIFY_NULL_FH_LEN;
>
> Please memset() the NULL_FH buffer to zero.
>
> > +
> > + goto success;
> > + }
> >
> > /*
> > * !gpf means preallocated variable size fh, but fh_len could
> > @@ -400,6 +414,7 @@ static int fanotify_encode_fh(struct fanotify_fh *fh, struct inode *inode,
> > if (!type || type == FILEID_INVALID || fh_len != dwords << 2)
> > goto out_err;
> >
> > +success:
> > fh->type = type;
> > fh->len = fh_len;
> >
> > @@ -529,7 +544,7 @@ static struct fanotify_event *fanotify_alloc_name_event(struct inode *id,
> > struct fanotify_info *info;
> > struct fanotify_fh *dfh, *ffh;
> > unsigned int dir_fh_len = fanotify_encode_fh_len(id);
> > - unsigned int child_fh_len = fanotify_encode_fh_len(child);
> > + unsigned int child_fh_len = child ? fanotify_encode_fh_len(child) : 0;
> > unsigned int size;
> >
> > size = sizeof(*fne) + FANOTIFY_FH_HDR_LEN + dir_fh_len;
> > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> > index c47a5a45c0d3..4cacea5fcaca 100644
> > --- a/fs/notify/fanotify/fanotify_user.c
> > +++ b/fs/notify/fanotify/fanotify_user.c
> > @@ -360,7 +360,10 @@ static int copy_info_to_user(__kernel_fsid_t *fsid, struct fanotify_fh *fh,
> > return -EFAULT;
> >
> > handle.handle_type = fh->type;
> > - handle.handle_bytes = fh_len;
> > +
> > + /* FILEID_INVALID handle type is reported without its f_handle. */
> > + if (fh->type != FILEID_INVALID)
> > + handle.handle_bytes = fh_len;
>
> I know I suggested those exact lines, but looking at the patch,
> I think it would be better to do:
> + if (fh->type != FILEID_INVALID)
> + fh_len = 0;
> handle.handle_bytes = fh_len;
>
> > if (copy_to_user(buf, &handle, sizeof(handle)))
> > return -EFAULT;
> >
> > @@ -369,20 +372,22 @@ static int copy_info_to_user(__kernel_fsid_t *fsid, struct fanotify_fh *fh,
> > if (WARN_ON_ONCE(len < fh_len))
> > return -EFAULT;
> >
> > - /*
> > - * For an inline fh and inline file name, copy through stack to exclude
> > - * the copy from usercopy hardening protections.
> > - */
> > - fh_buf = fanotify_fh_buf(fh);
> > - if (fh_len <= FANOTIFY_INLINE_FH_LEN) {
> > - memcpy(bounce, fh_buf, fh_len);
> > - fh_buf = bounce;
> > + if (fh->type != FILEID_INVALID) {
>
> ... and here: if (fh_len) {
>
> > + /*
> > + * For an inline fh and inline file name, copy through
> > + * stack to exclude the copy from usercopy hardening
> > + * protections.
> > + */
> > + fh_buf = fanotify_fh_buf(fh);
> > + if (fh_len <= FANOTIFY_INLINE_FH_LEN) {
> > + memcpy(bounce, fh_buf, fh_len);
> > + fh_buf = bounce;
> > + }
> > + if (copy_to_user(buf, fh_buf, fh_len))
> > + return -EFAULT;
> > + buf += fh_len;
> > + len -= fh_len;
> > }
> > - if (copy_to_user(buf, fh_buf, fh_len))
> > - return -EFAULT;
> > -
> > - buf += fh_len;
> > - len -= fh_len;
> >
> > if (name_len) {
> > /* Copy the filename with terminating null */
> > @@ -398,7 +403,7 @@ static int copy_info_to_user(__kernel_fsid_t *fsid, struct fanotify_fh *fh,
> > }
> >
> > /* Pad with 0's */
> > - WARN_ON_ONCE(len < 0 || len >= FANOTIFY_EVENT_ALIGN);
> > + WARN_ON_ONCE(len < 0);
>
> According to my calculations, FAN_FS_ERROR event with NULL_FH is expected
> to get here with len == 4, so you can change this to:
> WARN_ON_ONCE(len < 0 || len > FANOTIFY_EVENT_ALIGN);
>
> But first, I would like to get Jan's feedback on this concept of keeping
> unneeded 4 bytes zero padding in reported event in case of NULL_FH
> in order to keep the FID reporting code simpler.
Dunno, it still seems like quite some complications (simple ones but
non-trivial amount of them) for what is rather a corner case. What if we
*internally* propagated the information that there's no inode info with
FILEID_ROOT fh? That means: No changes to fanotify_encode_fh_len(),
fanotify_encode_fh(), or fanotify_alloc_name_event(). In
copy_info_to_user() we just mangle FILEID_ROOT to FILEID_INVALID and that's
all. No useless padding, no specialcasing of copying etc. Am I missing
something?
Honza
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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