vfs: strengthen checking of file range inputs to generic_remap_checks

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>
Signed-off-by: Dave Chinner <david@fromorbit.com>

1 files changed, 52 insertions, 32 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 47e6bfd45a91..84b7301e41a0 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2916,6 +2916,42 @@ struct page *read_cache_page_gfp(struct address_space *mapping,
 EXPORT_SYMBOL(read_cache_page_gfp);
 
 /*
+ * Don't operate on ranges the page cache doesn't support, and don't exceed the
+ * LFS limits.  If pos is under the limit it becomes a short access.  If it
+ * exceeds the limit we return -EFBIG.
+ */
+static int generic_access_check_limits(struct file *file, loff_t pos,
+                                       loff_t *count)
+{
+        struct inode *inode = file->f_mapping->host;
+        loff_t max_size = inode->i_sb->s_maxbytes;
+
+        if (!(file->f_flags & O_LARGEFILE))
+                max_size = MAX_NON_LFS;
+
+        if (unlikely(pos >= max_size))
+                return -EFBIG;
+        *count = min(*count, max_size - pos);
+        return 0;
+}
+
+static int generic_write_check_limits(struct file *file, loff_t pos,
+                                      loff_t *count)
+{
+        loff_t limit = rlimit(RLIMIT_FSIZE);
+
+        if (limit != RLIM_INFINITY) {
+                if (pos >= limit) {
+                        send_sig(SIGXFSZ, current, 0);
+                        return -EFBIG;
+                }
+                *count = min(*count, limit - pos);
+        }
+
+        return generic_access_check_limits(file, pos, count);
+}
+
+/*
  * Performs necessary checks before doing a write
  *
  * Can adjust writing position or amount of bytes to write.
@@ -2926,8 +2962,8 @@ 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 +2972,15 @@ 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);
-        }
-
-        /*
-         * 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;
+        count = iov_iter_count(from);
+        ret = generic_write_check_limits(file, iocb->ki_pos, &count);
+        if (ret)
+                return ret;
 
-        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 +3002,7 @@ 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 +3026,14 @@ 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.