ext4: fix undefined behavior in ext4_fill_flex_info()

Commit 503358ae01b70ce6909d19dd01287093f6b6271c ("ext4: avoid divide by
zero when trying to mount a corrupted file system") fixes CVE-2009-4307
by performing a sanity check on s_log_groups_per_flex, since it can be
set to a bogus value by an attacker.

	sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
	groups_per_flex = 1 << sbi->s_log_groups_per_flex;

	if (groups_per_flex < 2) { ... }

This patch fixes two potential issues in the previous commit.

1) The sanity check might only work on architectures like PowerPC.
On x86, 5 bits are used for the shifting amount.  That means, given a
large s_log_groups_per_flex value like 36, groups_per_flex = 1 << 36
is essentially 1 << 4 = 16, rather than 0.  This will bypass the check,
leaving s_log_groups_per_flex and groups_per_flex inconsistent.

2) The sanity check relies on undefined behavior, i.e., oversized shift.
A standard-confirming C compiler could rewrite the check in unexpected
ways.  Consider the following equivalent form, assuming groups_per_flex
is unsigned for simplicity.

	groups_per_flex = 1 << sbi->s_log_groups_per_flex;
	if (groups_per_flex == 0 || groups_per_flex == 1) {

We compile the code snippet using Clang 3.0 and GCC 4.6.  Clang will
completely optimize away the check groups_per_flex == 0, leaving the
patched code as vulnerable as the original.  GCC keeps the check, but
there is no guarantee that future versions will do the same.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org
static int ext4_fill_flex_info(struct super_block *sb)
{
	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	struct ext4_group_desc *gdp = NULL;
 	ext4_group_t flex_group_count;
 	ext4_group_t flex_group;
	unsigned int groups_per_flex = 0;
 	size_t size;
 	int i;
 
 	sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
	if (sbi->s_log_groups_per_flex < 1 || sbi->s_log_groups_per_flex > 31) {
 		sbi->s_log_groups_per_flex = 0;
 		return 1;
 	}
	groups_per_flex = 1 << sbi->s_log_groups_per_flex;
 
 	/* We allocate both existing and potentially added groups */
 	flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) +
			((le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) + 1) <<
			      EXT4_DESC_PER_BLOCK_BITS(sb))) / groups_per_flex;
	size = flex_group_count * sizeof(struct flex_groups);
	sbi->s_flex_groups = ext4_kvzalloc(size, GFP_KERNEL);
	if (sbi->s_flex_groups == NULL) {
		ext4_msg(sb, KERN_ERR, "not enough memory for %u flex groups",
			 flex_group_count);
		goto failed;
	}

	for (i = 0; i < sbi->s_groups_count; i++) {
		gdp = ext4_get_group_desc(sb, i, NULL);

		flex_group = ext4_flex_group(sbi, i);
		atomic_add(ext4_free_inodes_count(sb, gdp),
			   &sbi->s_flex_groups[flex_group].free_inodes);
		atomic_add(ext4_free_group_clusters(sb, gdp),
			   &sbi->s_flex_groups[flex_group].free_clusters);
		atomic_add(ext4_used_dirs_count(sb, gdp),
			   &sbi->s_flex_groups[flex_group].used_dirs);
	}

	return 1;
failed:
	return 0;
}

static int ext4_fill_flex_info(struct super_block *sb)
{
	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	struct ext4_group_desc *gdp = NULL;
 	ext4_group_t flex_group_count;
 	ext4_group_t flex_group;
	int groups_per_flex = 0;
 	size_t size;
 	int i;
 
 	sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
	groups_per_flex = 1 << sbi->s_log_groups_per_flex;
	if (groups_per_flex < 2) {
 		sbi->s_log_groups_per_flex = 0;
 		return 1;
 	}
 
 	/* We allocate both existing and potentially added groups */
 	flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) +
			((le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) + 1) <<
			      EXT4_DESC_PER_BLOCK_BITS(sb))) / groups_per_flex;
	size = flex_group_count * sizeof(struct flex_groups);
	sbi->s_flex_groups = ext4_kvzalloc(size, GFP_KERNEL);
	if (sbi->s_flex_groups == NULL) {
		ext4_msg(sb, KERN_ERR, "not enough memory for %u flex groups",
			 flex_group_count);
		goto failed;
	}

	for (i = 0; i < sbi->s_groups_count; i++) {
		gdp = ext4_get_group_desc(sb, i, NULL);

		flex_group = ext4_flex_group(sbi, i);
		atomic_add(ext4_free_inodes_count(sb, gdp),
			   &sbi->s_flex_groups[flex_group].free_inodes);
		atomic_add(ext4_free_group_clusters(sb, gdp),
			   &sbi->s_flex_groups[flex_group].free_clusters);
		atomic_add(ext4_used_dirs_count(sb, gdp),
			   &sbi->s_flex_groups[flex_group].used_dirs);
	}

	return 1;
failed:
	return 0;
}
