ext4: Fix race in ext4_inode_info.i_cached_extent (2ec0ae3a) · Commits · 戴 / test

If two CPU's simultaneously call ext4_ext_get_blocks() at the same time, there is nothing protecting the i_cached_extent structure from being used and updated at the same time. This could potentially cause the wrong location on disk to be read or written to, including potentially causing the corruption of the block group descriptors and/or inode table. This bug has been in the ext4 code since almost the very beginning of ext4's development. Fortunately once the data is stored in the page cache cache, ext4_get_blocks() doesn't need to be called, so trying to replicate this problem to the point where we could identify its root cause was *extremely* difficult. Many thanks to Kevin Shanahan for working over several months to be able to reproduce this easily so we could finally nail down the cause of the corruption. Signed-off-by:

"Theodore Ts'o" <tytso@mit.edu> Reviewed-by:

"Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>

fs/ext4/extents.c

+12 −5

Original line number	Diff line number	Diff line
		@@ -1841,11 +1841,13 @@ ext4_ext_put_in_cache(struct inode *inode, ext4_lblk_t block,
		{
		struct ext4_ext_cache *cex;
		BUG_ON(len == 0);
		spin_lock(&EXT4_I(inode)->i_block_reservation_lock);
		cex = &EXT4_I(inode)->i_cached_extent;
		cex->ec_type = type;
		cex->ec_block = block;
		cex->ec_len = len;
		cex->ec_start = start;
		spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
		}

		/*
		@@ -1902,12 +1904,17 @@ ext4_ext_in_cache(struct inode *inode, ext4_lblk_t block,
		struct ext4_extent *ex)
		{
		struct ext4_ext_cache *cex;
		int ret = EXT4_EXT_CACHE_NO;

		/*
		* We borrow i_block_reservation_lock to protect i_cached_extent
		*/
		spin_lock(&EXT4_I(inode)->i_block_reservation_lock);
		cex = &EXT4_I(inode)->i_cached_extent;

		/* has cache valid data? */
		if (cex->ec_type == EXT4_EXT_CACHE_NO)
		return EXT4_EXT_CACHE_NO;
		goto errout;

		BUG_ON(cex->ec_type != EXT4_EXT_CACHE_GAP &&
		cex->ec_type != EXT4_EXT_CACHE_EXTENT);
		@@ -1918,11 +1925,11 @@ ext4_ext_in_cache(struct inode *inode, ext4_lblk_t block,
		ext_debug("%u cached by %u:%u:%llu\n",
		block,
		cex->ec_block, cex->ec_len, cex->ec_start);
		return cex->ec_type;
		ret = cex->ec_type;
		}

		/* not in cache */
		return EXT4_EXT_CACHE_NO;
		errout:
		spin_unlock(&EXT4_I(inode)->i_block_reservation_lock);
		return ret;
		}

		/*

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