Re: [PATCH 1/8] xfs: hide log iovec alignment constraints

Darrick J. Wong
2022-04-27T03:14:52Z

On Wed, Apr 27, 2022 at 12:22:52PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Callers currently have to round out the size of buffers to match the
> aligment constraints of log iovecs and xlog_write(). They should not
> need to know this detail, so introduce a new function to calculate
> the iovec length (for use in ->iop_size implementations). Also
> modify xlog_finish_iovec() to round up the length to the correct
> alignment so the callers don't need to do this, either.
>
> Convert the only user - inode forks - of this alignment rounding to
> use the new interface.

Hmm. So currently, we require that the inode fork buffer be rounded up
to the next 4 bytes, and then I guess the log will copy that into the
log iovec? IOWs, if we have a 37-byte data fork, we'll allocate a 40
byte buffer for the xfs_ifork, and the log will copy all 40 bytes into a
40 byte iovec.

Now it looks like we'd allocate a 37-byte buffer for the xfs_ifork, but
the log iovec will still be 40 bytes. So ... do we copy 37 bytes out of
the ifork buffer and zero the last 3 bytes in the iovec? Does we leak
kernel memory in those last 3 bytes? Or do we copy 40 bytes and
overrun?

It sorta looks like (at least for the local format case) xlog_copy_iovec
will copy 37 bytes and leave the last 3 bytes of the iovec in whatever
state it was in previously. Is that zeroed? Because it then looks like
xlog_finish_iovec will round that 37 up to 40.

(FWIW I'm just checking for kernel memory exposure vectors here.)

--D

> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/libxfs/xfs_inode_fork.c | 12 +++---------
> fs/xfs/xfs_inode_item.c | 25 +++++++------------------
> fs/xfs/xfs_inode_item_recover.c | 4 ++--
> fs/xfs/xfs_log.h | 23 +++++++++++++++++++++++
> 4 files changed, 35 insertions(+), 29 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_inode_fork.c b/fs/xfs/libxfs/xfs_inode_fork.c
> index 9aee4a1e2fe9..1a4cdf550f6d 100644
> --- a/fs/xfs/libxfs/xfs_inode_fork.c
> +++ b/fs/xfs/libxfs/xfs_inode_fork.c
> @@ -36,7 +36,7 @@ xfs_init_local_fork(
> int64_t size)
> {
> struct xfs_ifork *ifp = XFS_IFORK_PTR(ip, whichfork);
> - int mem_size = size, real_size = 0;
> + int mem_size = size;
> bool zero_terminate;
>
> /*
> @@ -50,8 +50,7 @@ xfs_init_local_fork(
> mem_size++;
>
> if (size) {
> - real_size = roundup(mem_size, 4);
> - ifp->if_u1.if_data = kmem_alloc(real_size, KM_NOFS);
> + ifp->if_u1.if_data = kmem_alloc(mem_size, KM_NOFS);
> memcpy(ifp->if_u1.if_data, data, size);
> if (zero_terminate)
> ifp->if_u1.if_data[size] = '\0';
> @@ -497,12 +496,7 @@ xfs_idata_realloc(
> return;
> }
>
> - /*
> - * For inline data, the underlying buffer must be a multiple of 4 bytes
> - * in size so that it can be logged and stay on word boundaries.
> - * We enforce that here.
> - */
> - ifp->if_u1.if_data = krealloc(ifp->if_u1.if_data, roundup(new_size, 4),
> + ifp->if_u1.if_data = krealloc(ifp->if_u1.if_data, new_size,
> GFP_NOFS | __GFP_NOFAIL);
> ifp->if_bytes = new_size;
> }
> diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
> index 00733a18ccdc..721def0639fd 100644
> --- a/fs/xfs/xfs_inode_item.c
> +++ b/fs/xfs/xfs_inode_item.c
> @@ -71,7 +71,7 @@ xfs_inode_item_data_fork_size(
> case XFS_DINODE_FMT_LOCAL:
> if ((iip->ili_fields & XFS_ILOG_DDATA) &&
> ip->i_df.if_bytes > 0) {
> - *nbytes += roundup(ip->i_df.if_bytes, 4);
> + *nbytes += xlog_calc_iovec_len(ip->i_df.if_bytes);
> *nvecs += 1;
> }
> break;
> @@ -112,7 +112,7 @@ xfs_inode_item_attr_fork_size(
> case XFS_DINODE_FMT_LOCAL:
> if ((iip->ili_fields & XFS_ILOG_ADATA) &&
> ip->i_afp->if_bytes > 0) {
> - *nbytes += roundup(ip->i_afp->if_bytes, 4);
> + *nbytes += xlog_calc_iovec_len(ip->i_afp->if_bytes);
> *nvecs += 1;
> }
> break;
> @@ -204,17 +204,12 @@ xfs_inode_item_format_data_fork(
> ~(XFS_ILOG_DEXT | XFS_ILOG_DBROOT | XFS_ILOG_DEV);
> if ((iip->ili_fields & XFS_ILOG_DDATA) &&
> ip->i_df.if_bytes > 0) {
> - /*
> - * Round i_bytes up to a word boundary.
> - * The underlying memory is guaranteed
> - * to be there by xfs_idata_realloc().
> - */
> - data_bytes = roundup(ip->i_df.if_bytes, 4);
> ASSERT(ip->i_df.if_u1.if_data != NULL);
> ASSERT(ip->i_disk_size > 0);
> xlog_copy_iovec(lv, vecp, XLOG_REG_TYPE_ILOCAL,
> - ip->i_df.if_u1.if_data, data_bytes);
> - ilf->ilf_dsize = (unsigned)data_bytes;
> + ip->i_df.if_u1.if_data,
> + ip->i_df.if_bytes);
> + ilf->ilf_dsize = (unsigned)ip->i_df.if_bytes;
> ilf->ilf_size++;
> } else {
> iip->ili_fields &= ~XFS_ILOG_DDATA;
> @@ -288,17 +283,11 @@ xfs_inode_item_format_attr_fork(
>
> if ((iip->ili_fields & XFS_ILOG_ADATA) &&
> ip->i_afp->if_bytes > 0) {
> - /*
> - * Round i_bytes up to a word boundary.
> - * The underlying memory is guaranteed
> - * to be there by xfs_idata_realloc().
> - */
> - data_bytes = roundup(ip->i_afp->if_bytes, 4);
> ASSERT(ip->i_afp->if_u1.if_data != NULL);
> xlog_copy_iovec(lv, vecp, XLOG_REG_TYPE_IATTR_LOCAL,
> ip->i_afp->if_u1.if_data,
> - data_bytes);
> - ilf->ilf_asize = (unsigned)data_bytes;
> + ip->i_afp->if_bytes);
> + ilf->ilf_asize = (unsigned)ip->i_afp->if_bytes;
> ilf->ilf_size++;
> } else {
> iip->ili_fields &= ~XFS_ILOG_ADATA;
> diff --git a/fs/xfs/xfs_inode_item_recover.c b/fs/xfs/xfs_inode_item_recover.c
> index 6d44f5fd6d7e..d28ffaebd067 100644
> --- a/fs/xfs/xfs_inode_item_recover.c
> +++ b/fs/xfs/xfs_inode_item_recover.c
> @@ -462,7 +462,7 @@ xlog_recover_inode_commit_pass2(
> ASSERT(in_f->ilf_size <= 4);
> ASSERT((in_f->ilf_size == 3) || (fields & XFS_ILOG_AFORK));
> ASSERT(!(fields & XFS_ILOG_DFORK) ||
> - (len == in_f->ilf_dsize));
> + (len == xlog_calc_iovec_len(in_f->ilf_dsize)));
>
> switch (fields & XFS_ILOG_DFORK) {
> case XFS_ILOG_DDATA:
> @@ -497,7 +497,7 @@ xlog_recover_inode_commit_pass2(
> }
> len = item->ri_buf[attr_index].i_len;
> src = item->ri_buf[attr_index].i_addr;
> - ASSERT(len == in_f->ilf_asize);
> + ASSERT(len == xlog_calc_iovec_len(in_f->ilf_asize));
>
> switch (in_f->ilf_fields & XFS_ILOG_AFORK) {
> case XFS_ILOG_ADATA:
> diff --git a/fs/xfs/xfs_log.h b/fs/xfs/xfs_log.h
> index 8dafe8f771c7..d9aebca3f971 100644
> --- a/fs/xfs/xfs_log.h
> +++ b/fs/xfs/xfs_log.h
> @@ -21,6 +21,17 @@ struct xfs_log_vec {
>
> #define XFS_LOG_VEC_ORDERED (-1)
>
> +/*
> + * Calculate the log iovec length for a given user buffer length. Intended to be
> + * used by ->iop_size implementations when sizing buffers of arbitrary
> + * alignments.
> + */
> +static inline int
> +xlog_calc_iovec_len(int len)
> +{
> + return roundup(len, 4);
> +}
> +
> void *xlog_prepare_iovec(struct xfs_log_vec *lv, struct xfs_log_iovec **vecp,
> uint type);
>
> @@ -29,6 +40,12 @@ xlog_finish_iovec(struct xfs_log_vec *lv, struct xfs_log_iovec *vec, int len)
> {
> struct xlog_op_header *oph = vec->i_addr;
>
> + /*
> + * Always round up the length to the correct alignment so callers don't
> + * need to know anything about this log vec layout requirement.
> + */
> + len = xlog_calc_iovec_len(len);
> +
> /* opheader tracks payload length, logvec tracks region length */
> oph->oh_len = cpu_to_be32(len);
>
> @@ -36,8 +53,14 @@ xlog_finish_iovec(struct xfs_log_vec *lv, struct xfs_log_iovec *vec, int len)
> lv->lv_buf_len += len;
> lv->lv_bytes += len;
> vec->i_len = len;
> +
> + /* Catch buffer overruns */
> + ASSERT((void *)lv->lv_buf + lv->lv_bytes <= (void *)lv + lv->lv_size);
> }
>
> +/*
> + * Copy the amount of data requested by the caller into a new log iovec.
> + */
> static inline void *
> xlog_copy_iovec(struct xfs_log_vec *lv, struct xfs_log_iovec **vecp,
> uint type, void *data, int len)
> --
> 2.35.1
>