[PATCH 04/29] vfs: strengthen checking of file range inputs to generic_remap_checks

Darrick J. Wong
2018-10-18T06:42:48Z

File range remapping, if allowed to run past the destination file's EOF,
is an optimization on a regular file write. Regular file writes that
extend the file length are subject to various constraints which are not
checked by range cloning.

This is a correctness problem because we're never allowed to touch
ranges that the page cache can't support (s_maxbytes); we're not
supposed to deal with large offsets (MAX_NON_LFS) if O_LARGEFILE isn't
set; and we must obey resource limits (RLIMIT_FSIZE).

Therefore, add these checks to the new generic_remap_checks function so
that we curtail unexpected behavior.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
mm/filemap.c | 91 ++++++++++++++++++++++++++++++++++++++--------------------
1 file changed, 59 insertions(+), 32 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 47e6bfd45a91..08ad210fee49 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2915,6 +2915,49 @@ struct page *read_cache_page_gfp(struct address_space *mapping,
}
EXPORT_SYMBOL(read_cache_page_gfp);

+static int generic_access_check_limits(struct file *file, loff_t pos,
+ loff_t *count)
+{
+ struct inode *inode = file->f_mapping->host;
+
+ /* Don't exceed the LFS limits. */
+ if (unlikely(pos + *count > MAX_NON_LFS &&
+ !(file->f_flags & O_LARGEFILE))) {
+ if (pos >= MAX_NON_LFS)
+ return -EFBIG;
+ *count = min(*count, (loff_t)MAX_NON_LFS - pos);
+ }
+
+ /*
+ * Don't operate on ranges the page cache doesn't support.
+ *
+ * If we have written data it becomes a short write. If we have
+ * exceeded without writing data we send a signal and return EFBIG.
+ * Linus frestrict idea will clean these up nicely..
+ */
+ if (unlikely(pos >= inode->i_sb->s_maxbytes))
+ return -EFBIG;
+
+ *count = min(*count, inode->i_sb->s_maxbytes - pos);
+ return 0;
+}
+
+static int generic_write_check_limits(struct file *file, loff_t pos,
+ loff_t *count)
+{
+ unsigned long limit = rlimit(RLIMIT_FSIZE);
+
+ if (limit != RLIM_INFINITY) {
+ if (pos >= limit) {
+ send_sig(SIGXFSZ, current, 0);
+ return -EFBIG;
+ }
+ *count = min(*count, (loff_t)limit - pos);
+ }
+
+ return generic_access_check_limits(file, pos, count);
+}
+
/*
* Performs necessary checks before doing a write
*
@@ -2926,8 +2969,8 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
{
struct file *file = iocb->ki_filp;
struct inode *inode = file->f_mapping->host;
- unsigned long limit = rlimit(RLIMIT_FSIZE);
- loff_t pos;
+ loff_t count;
+ int ret;

if (!iov_iter_count(from))
return 0;
@@ -2936,40 +2979,15 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
if (iocb->ki_flags & IOCB_APPEND)
iocb->ki_pos = i_size_read(inode);

- pos = iocb->ki_pos;
-
if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
return -EINVAL;

- if (limit != RLIM_INFINITY) {
- if (iocb->ki_pos >= limit) {
- send_sig(SIGXFSZ, current, 0);
- return -EFBIG;
- }
- iov_iter_truncate(from, limit - (unsigned long)pos);
- }
+ count = iov_iter_count(from);
+ ret = generic_write_check_limits(file, iocb->ki_pos, &count);
+ if (ret)
+ return ret;

- /*
- * LFS rule
- */
- if (unlikely(pos + iov_iter_count(from) > MAX_NON_LFS &&
- !(file->f_flags & O_LARGEFILE))) {
- if (pos >= MAX_NON_LFS)
- return -EFBIG;
- iov_iter_truncate(from, MAX_NON_LFS - (unsigned long)pos);
- }
-
- /*
- * Are we about to exceed the fs block limit ?
- *
- * If we have written data it becomes a short write. If we have
- * exceeded without writing data we send a signal and return EFBIG.
- * Linus frestrict idea will clean these up nicely..
- */
- if (unlikely(pos >= inode->i_sb->s_maxbytes))
- return -EFBIG;
-
- iov_iter_truncate(from, inode->i_sb->s_maxbytes - pos);
+ iov_iter_truncate(from, count);
return iov_iter_count(from);
}
EXPORT_SYMBOL(generic_write_checks);
@@ -2991,6 +3009,7 @@ int generic_remap_checks(struct file *file_in, loff_t pos_in,
uint64_t bcount;
loff_t size_in, size_out;
loff_t bs = inode_out->i_sb->s_blocksize;
+ int ret;

/* The start of both ranges must be aligned to an fs block. */
if (!IS_ALIGNED(pos_in, bs) || !IS_ALIGNED(pos_out, bs))
@@ -3014,6 +3033,14 @@ int generic_remap_checks(struct file *file_in, loff_t pos_in,
return -EINVAL;
count = min(count, size_in - (uint64_t)pos_in);

+ ret = generic_access_check_limits(file_in, pos_in, &count);
+ if (ret)
+ return ret;
+
+ ret = generic_write_check_limits(file_out, pos_out, &count);
+ if (ret)
+ return ret;
+
/*
* If the user wanted us to link to the infile's EOF, round up to the
* next block boundary for this check.