1 files changed, 32 insertions, 14 deletions

diff --git a/fs/verity/enable.c b/fs/verity/enable.c
index e13db6507b38..fc4c50e5219d 100644
--- a/fs/verity/enable.c
+++ b/fs/verity/enable.c
@@ -8,12 +8,12 @@
 #include "fsverity_private.h"
 
 #include <linux/mount.h>
-#include <linux/pagemap.h>
 #include <linux/sched/signal.h>
 #include <linux/uaccess.h>
 
 struct block_buffer {
 	u32 filled;
+	bool is_root_hash;
 	u8 *data;
 };
 
@@ -25,6 +25,14 @@ static int hash_one_block(struct inode *inode,
 	struct block_buffer *next = cur + 1;
 	int err;
 
+	/*
+	 * Safety check to prevent a buffer overflow in case of a filesystem bug
+	 * that allows the file size to change despite deny_write_access(), or a
+	 * bug in the Merkle tree logic itself
+	 */
+	if (WARN_ON_ONCE(next->is_root_hash && next->filled != 0))
+		return -EINVAL;
+
 	/* Zero-pad the block if it's shorter than the block size. */
 	memset(&cur->data[cur->filled], 0, params->block_size - cur->filled);
 
@@ -98,6 +106,7 @@ static int build_merkle_tree(struct file *filp,
 		}
 	}
 	buffers[num_levels].data = root_hash;
+	buffers[num_levels].is_root_hash = true;
 
 	BUILD_BUG_ON(sizeof(level_offset) != sizeof(params->level_start));
 	memcpy(level_offset, params->level_start, sizeof(level_offset));
@@ -166,7 +175,7 @@ static int build_merkle_tree(struct file *filp,
 		}
 	}
 	/* The root hash was filled by the last call to hash_one_block(). */
-	if (WARN_ON(buffers[num_levels].filled != params->digest_size)) {
+	if (WARN_ON_ONCE(buffers[num_levels].filled != params->digest_size)) {
 		err = -EINVAL;
 		goto out;
 	}
@@ -278,7 +287,7 @@ static int enable_verity(struct file *filp,
 		fsverity_err(inode, "%ps() failed with err %d",
 			     vops->end_enable_verity, err);
 		fsverity_free_info(vi);
-	} else if (WARN_ON(!IS_VERITY(inode))) {
+	} else if (WARN_ON_ONCE(!IS_VERITY(inode))) {
 		err = -EINVAL;
 		fsverity_free_info(vi);
 	} else {
@@ -348,6 +357,13 @@ int fsverity_ioctl_enable(struct file *filp, const void __user *uarg)
 	err = file_permission(filp, MAY_WRITE);
 	if (err)
 		return err;
+	/*
+	 * __kernel_read() is used while building the Merkle tree.  So, we can't
+	 * allow file descriptors that were opened for ioctl access only, using
+	 * the special nonstandard access mode 3.  O_RDONLY only, please!
+	 */
+	if (!(filp->f_mode & FMODE_READ))
+		return -EBADF;
 
 	if (IS_APPEND(inode))
 		return -EPERM;
@@ -367,25 +383,27 @@ int fsverity_ioctl_enable(struct file *filp, const void __user *uarg)
 		goto out_drop_write;
 
 	err = enable_verity(filp, &arg);
-	if (err)
-		goto out_allow_write_access;
 
 	/*
-	 * Some pages of the file may have been evicted from pagecache after
-	 * being used in the Merkle tree construction, then read into pagecache
-	 * again by another process reading from the file concurrently.  Since
-	 * these pages didn't undergo verification against the file digest which
-	 * fs-verity now claims to be enforcing, we have to wipe the pagecache
-	 * to ensure that all future reads are verified.
+	 * We no longer drop the inode's pagecache after enabling verity.  This
+	 * used to be done to try to avoid a race condition where pages could be
+	 * evicted after being used in the Merkle tree construction, then
+	 * re-instantiated by a concurrent read.  Such pages are unverified, and
+	 * the backing storage could have filled them with different content, so
+	 * they shouldn't be used to fulfill reads once verity is enabled.
+	 *
+	 * But, dropping the pagecache has a big performance impact, and it
+	 * doesn't fully solve the race condition anyway.  So for those reasons,
+	 * and also because this race condition isn't very important relatively
+	 * speaking (especially for small-ish files, where the chance of a page
+	 * being used, evicted, *and* re-instantiated all while enabling verity
+	 * is quite small), we no longer drop the inode's pagecache.
 	 */
-	filemap_write_and_wait(inode->i_mapping);
-	invalidate_inode_pages2(inode->i_mapping);
 
 	/*
 	 * allow_write_access() is needed to pair with deny_write_access().
 	 * Regardless, the filesystem won't allow writing to verity files.
 	 */
-out_allow_write_access:
 	allow_write_access(filp);
 out_drop_write:
 	mnt_drop_write_file(filp);