[PATCH 3/3] NTFS: mft: Remove VLA usage

From: Kees Cook
Date: Tue Jun 26 2018 - 13:29:43 EST


In the quest to remove all stack VLA usage from the kernel[1], this
allocates the maximum size stack buffer. Existing checks already require
that blocksize >= NTFS_BLOCK_SIZE and mft_record_size <= PAGE_SIZE, so
max_bhs can be at most PAGE_SIZE / NTFS_BLOCK_SIZE. Sanity checks are
added for robustness.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@xxxxxxxxxxxxxx

Cc: Anton Altaparmakov <anton@xxxxxxxxxx>
Cc: linux-ntfs-dev@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
fs/ntfs/mft.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/ntfs/mft.c b/fs/ntfs/mft.c
index 32c523cf5a2d..fb14d17666c8 100644
--- a/fs/ntfs/mft.c
+++ b/fs/ntfs/mft.c
@@ -35,6 +35,8 @@
#include "mft.h"
#include "ntfs.h"

+#define MAX_BHS (PAGE_SIZE / NTFS_BLOCK_SIZE)
+
/**
* map_mft_record_page - map the page in which a specific mft record resides
* @ni: ntfs inode whose mft record page to map
@@ -469,7 +471,7 @@ int ntfs_sync_mft_mirror(ntfs_volume *vol, const unsigned long mft_no,
struct page *page;
unsigned int blocksize = vol->sb->s_blocksize;
int max_bhs = vol->mft_record_size / blocksize;
- struct buffer_head *bhs[max_bhs];
+ struct buffer_head *bhs[MAX_BHS];
struct buffer_head *bh, *head;
u8 *kmirr;
runlist_element *rl;
@@ -479,6 +481,8 @@ int ntfs_sync_mft_mirror(ntfs_volume *vol, const unsigned long mft_no,

ntfs_debug("Entering for inode 0x%lx.", mft_no);
BUG_ON(!max_bhs);
+ if (WARN_ON(max_bhs > MAX_BHS))
+ return -EINVAL;
if (unlikely(!vol->mftmirr_ino)) {
/* This could happen during umount... */
err = ntfs_sync_mft_mirror_umount(vol, mft_no, m);
@@ -674,7 +678,7 @@ int write_mft_record_nolock(ntfs_inode *ni, MFT_RECORD *m, int sync)
unsigned int blocksize = vol->sb->s_blocksize;
unsigned char blocksize_bits = vol->sb->s_blocksize_bits;
int max_bhs = vol->mft_record_size / blocksize;
- struct buffer_head *bhs[max_bhs];
+ struct buffer_head *bhs[MAX_BHS];
struct buffer_head *bh, *head;
runlist_element *rl;
unsigned int block_start, block_end, m_start, m_end;
@@ -684,6 +688,10 @@ int write_mft_record_nolock(ntfs_inode *ni, MFT_RECORD *m, int sync)
BUG_ON(NInoAttr(ni));
BUG_ON(!max_bhs);
BUG_ON(!PageLocked(page));
+ if (WARN_ON(max_bhs > MAX_BHS)) {
+ err = -EINVAL;
+ goto err_out;
+ }
/*
* If the ntfs_inode is clean no need to do anything. If it is dirty,
* mark it as clean now so that it can be redirtied later on if needed.
--
2.17.1