[PATCH 06/25] vfs: strengthen checking of file range inputs to generic_remap_checks

Darrick J. Wong
2018-10-10T00:11:20Z

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>
---
mm/filemap.c | 39 +++++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 14041a8468ba..59056bd9c58a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2974,6 +2974,27 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
}
EXPORT_SYMBOL(generic_write_checks);

+static int
+generic_remap_check_limits(struct file *file, loff_t pos, uint64_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, MAX_NON_LFS - (uint64_t)pos);
+ }
+
+ /* Don't operate on ranges the page cache doesn't support. */
+ if (unlikely(pos >= inode->i_sb->s_maxbytes))
+ return -EFBIG;
+
+ *count = min(*count, inode->i_sb->s_maxbytes - (uint64_t)pos);
+ return 0;
+}
+
/*
* Performs necessary checks before doing a clone.
*
@@ -2992,6 +3013,7 @@ 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))
@@ -3015,6 +3037,23 @@ int generic_remap_checks(struct file *file_in, loff_t pos_in,
return -EINVAL;
count = min(count, size_in - (uint64_t)pos_in);

+ /* Don't exceed RLMIT_FSIZE in the file we're writing into. */
+ if (limit != RLIM_INFINITY) {
+ if (pos_out >= limit) {
+ send_sig(SIGXFSZ, current, 0);
+ return -EFBIG;
+ }
+ count = min(count, limit - (uint64_t)pos_out);
+ }
+
+ ret = generic_remap_check_limits(file_in, pos_in, &count);
+ if (ret)
+ return ret;
+
+ ret = generic_remap_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.