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Message-ID: <20220815145547.hsufnubt7rgmqnxk@quack3>
Date: Mon, 15 Aug 2022 16:55:47 +0200
From: Jan Kara <jack@...e.cz>
To: Dan Carpenter <dan.carpenter@...cle.com>
Cc: jack@...e.cz, linux-ext4@...r.kernel.org
Subject: Re: [bug report] ext4: fix race when reusing xattr blocks
Hi Dan!
On Thu 04-08-22 12:02:03, Dan Carpenter wrote:
> [ This code is above my pay grade. The cache is refcounted so it might
> not be freed. But hopefully the patch is recent enough that the details
> are still top of the head for you to review it quickly? Hopefully, no
> big deal if it's a false positive. These reports are one time only.
> -dan ]
>
> Hello Jan Kara,
>
> The patch 132991ed2882: "ext4: fix race when reusing xattr blocks"
> from Jul 12, 2022, leads to the following Smatch static checker
> warning:
>
> fs/ext4/xattr.c:2038 ext4_xattr_block_set()
> warn: 'ea_block_cache' was already freed.
Hum, so I'm not sure why smatch thinks this. The 'ce' entry has been looked
up by ext4_xattr_block_cache_find() and if that function returns entry, it
grabs a reference to that entry which protects 'ce' from being freed. And I
don't see how we could drop the 'ce' reference before reaching that
mb_cache_entry_put() call which is the one smatch complains about. Oh, now
I see it is not complaining about 'ce' entry but rather about
ea_block_cache. That is definitely safe because cache is destroyed only on
unmount / failed mount at which time there should not be other processes
manipulating xattrs on that filesystem. So AFAICT this is a false positive.
Honza
>
> fs/ext4/xattr.c
> 1850 static int
> 1851 ext4_xattr_block_set(handle_t *handle, struct inode *inode,
> 1852 struct ext4_xattr_info *i,
> 1853 struct ext4_xattr_block_find *bs)
> 1854 {
> 1855 struct super_block *sb = inode->i_sb;
> 1856 struct buffer_head *new_bh = NULL;
> 1857 struct ext4_xattr_search s_copy = bs->s;
> 1858 struct ext4_xattr_search *s = &s_copy;
> 1859 struct mb_cache_entry *ce = NULL;
> 1860 int error = 0;
> 1861 struct mb_cache *ea_block_cache = EA_BLOCK_CACHE(inode);
> 1862 struct inode *ea_inode = NULL, *tmp_inode;
> 1863 size_t old_ea_inode_quota = 0;
> 1864 unsigned int ea_ino;
> 1865
> 1866
> 1867 #define header(x) ((struct ext4_xattr_header *)(x))
> 1868
> 1869 if (s->base) {
> 1870 int offset = (char *)s->here - bs->bh->b_data;
> 1871
> 1872 BUFFER_TRACE(bs->bh, "get_write_access");
> 1873 error = ext4_journal_get_write_access(handle, sb, bs->bh,
> 1874 EXT4_JTR_NONE);
> 1875 if (error)
> 1876 goto cleanup;
> 1877 lock_buffer(bs->bh);
> 1878
> 1879 if (header(s->base)->h_refcount == cpu_to_le32(1)) {
> 1880 __u32 hash = le32_to_cpu(BHDR(bs->bh)->h_hash);
> 1881
> 1882 /*
> 1883 * This must happen under buffer lock for
> 1884 * ext4_xattr_block_set() to reliably detect modified
> 1885 * block
> 1886 */
> 1887 if (ea_block_cache) {
> 1888 struct mb_cache_entry *oe;
> 1889
> 1890 oe = mb_cache_entry_delete_or_get(ea_block_cache,
> 1891 hash, bs->bh->b_blocknr);
>
> "ea_block_cache" potentially freed here?
>
> 1892 if (oe) {
> 1893 /*
> 1894 * Xattr block is getting reused. Leave
> 1895 * it alone.
> 1896 */
> 1897 mb_cache_entry_put(ea_block_cache, oe);
>
> Also here?
>
> 1898 goto clone_block;
> 1899 }
> 1900 }
> 1901 ea_bdebug(bs->bh, "modifying in-place");
> 1902 error = ext4_xattr_set_entry(i, s, handle, inode,
> 1903 true /* is_block */);
> 1904 ext4_xattr_block_csum_set(inode, bs->bh);
> 1905 unlock_buffer(bs->bh);
> 1906 if (error == -EFSCORRUPTED)
> 1907 goto bad_block;
> 1908 if (!error)
> 1909 error = ext4_handle_dirty_metadata(handle,
> 1910 inode,
> 1911 bs->bh);
> 1912 if (error)
> 1913 goto cleanup;
> 1914 goto inserted;
> 1915 }
> 1916 clone_block:
> 1917 unlock_buffer(bs->bh);
> 1918 ea_bdebug(bs->bh, "cloning");
> 1919 s->base = kmemdup(BHDR(bs->bh), bs->bh->b_size, GFP_NOFS);
> 1920 error = -ENOMEM;
> 1921 if (s->base == NULL)
> 1922 goto cleanup;
> 1923 s->first = ENTRY(header(s->base)+1);
> 1924 header(s->base)->h_refcount = cpu_to_le32(1);
> 1925 s->here = ENTRY(s->base + offset);
> 1926 s->end = s->base + bs->bh->b_size;
> 1927
> 1928 /*
> 1929 * If existing entry points to an xattr inode, we need
> 1930 * to prevent ext4_xattr_set_entry() from decrementing
> 1931 * ref count on it because the reference belongs to the
> 1932 * original block. In this case, make the entry look
> 1933 * like it has an empty value.
> 1934 */
> 1935 if (!s->not_found && s->here->e_value_inum) {
> 1936 ea_ino = le32_to_cpu(s->here->e_value_inum);
> 1937 error = ext4_xattr_inode_iget(inode, ea_ino,
> 1938 le32_to_cpu(s->here->e_hash),
> 1939 &tmp_inode);
> 1940 if (error)
> 1941 goto cleanup;
> 1942
> 1943 if (!ext4_test_inode_state(tmp_inode,
> 1944 EXT4_STATE_LUSTRE_EA_INODE)) {
> 1945 /*
> 1946 * Defer quota free call for previous
> 1947 * inode until success is guaranteed.
> 1948 */
> 1949 old_ea_inode_quota = le32_to_cpu(
> 1950 s->here->e_value_size);
> 1951 }
> 1952 iput(tmp_inode);
> 1953
> 1954 s->here->e_value_inum = 0;
> 1955 s->here->e_value_size = 0;
> 1956 }
> 1957 } else {
> 1958 /* Allocate a buffer where we construct the new block. */
> 1959 s->base = kzalloc(sb->s_blocksize, GFP_NOFS);
> 1960 error = -ENOMEM;
> 1961 if (s->base == NULL)
> 1962 goto cleanup;
> 1963 header(s->base)->h_magic = cpu_to_le32(EXT4_XATTR_MAGIC);
> 1964 header(s->base)->h_blocks = cpu_to_le32(1);
> 1965 header(s->base)->h_refcount = cpu_to_le32(1);
> 1966 s->first = ENTRY(header(s->base)+1);
> 1967 s->here = ENTRY(header(s->base)+1);
> 1968 s->end = s->base + sb->s_blocksize;
> 1969 }
> 1970
> 1971 error = ext4_xattr_set_entry(i, s, handle, inode, true /* is_block */);
> 1972 if (error == -EFSCORRUPTED)
> 1973 goto bad_block;
> 1974 if (error)
> 1975 goto cleanup;
> 1976
> 1977 if (i->value && s->here->e_value_inum) {
> 1978 /*
> 1979 * A ref count on ea_inode has been taken as part of the call to
> 1980 * ext4_xattr_set_entry() above. We would like to drop this
> 1981 * extra ref but we have to wait until the xattr block is
> 1982 * initialized and has its own ref count on the ea_inode.
> 1983 */
> 1984 ea_ino = le32_to_cpu(s->here->e_value_inum);
> 1985 error = ext4_xattr_inode_iget(inode, ea_ino,
> 1986 le32_to_cpu(s->here->e_hash),
> 1987 &ea_inode);
> 1988 if (error) {
> 1989 ea_inode = NULL;
> 1990 goto cleanup;
> 1991 }
> 1992 }
> 1993
> 1994 inserted:
> 1995 if (!IS_LAST_ENTRY(s->first)) {
> 1996 new_bh = ext4_xattr_block_cache_find(inode, header(s->base),
> 1997 &ce);
> 1998 if (new_bh) {
> 1999 /* We found an identical block in the cache. */
> 2000 if (new_bh == bs->bh)
> 2001 ea_bdebug(new_bh, "keeping");
> 2002 else {
> 2003 u32 ref;
> 2004
> 2005 WARN_ON_ONCE(dquot_initialize_needed(inode));
> 2006
> 2007 /* The old block is released after updating
> 2008 the inode. */
> 2009 error = dquot_alloc_block(inode,
> 2010 EXT4_C2B(EXT4_SB(sb), 1));
> 2011 if (error)
> 2012 goto cleanup;
> 2013 BUFFER_TRACE(new_bh, "get_write_access");
> 2014 error = ext4_journal_get_write_access(
> 2015 handle, sb, new_bh,
> 2016 EXT4_JTR_NONE);
> 2017 if (error)
> 2018 goto cleanup_dquot;
> 2019 lock_buffer(new_bh);
> 2020 /*
> 2021 * We have to be careful about races with
> 2022 * adding references to xattr block. Once we
> 2023 * hold buffer lock xattr block's state is
> 2024 * stable so we can check the additional
> 2025 * reference fits.
> 2026 */
> 2027 ref = le32_to_cpu(BHDR(new_bh)->h_refcount) + 1;
> 2028 if (ref > EXT4_XATTR_REFCOUNT_MAX) {
> 2029 /*
> 2030 * Undo everything and check mbcache
> 2031 * again.
> 2032 */
> 2033 unlock_buffer(new_bh);
> 2034 dquot_free_block(inode,
> 2035 EXT4_C2B(EXT4_SB(sb),
> 2036 1));
> 2037 brelse(new_bh);
> --> 2038 mb_cache_entry_put(ea_block_cache, ce);
>
> Warning.
>
> 2039 ce = NULL;
> 2040 new_bh = NULL;
> 2041 goto inserted;
> 2042 }
> 2043 BHDR(new_bh)->h_refcount = cpu_to_le32(ref);
> 2044 if (ref == EXT4_XATTR_REFCOUNT_MAX)
> 2045 ce->e_reusable = 0;
> 2046 ea_bdebug(new_bh, "reusing; refcount now=%d",
> 2047 ref);
> 2048 ext4_xattr_block_csum_set(inode, new_bh);
> 2049 unlock_buffer(new_bh);
> 2050 error = ext4_handle_dirty_metadata(handle,
> 2051 inode,
> 2052 new_bh);
> 2053 if (error)
> 2054 goto cleanup_dquot;
> 2055 }
> 2056 mb_cache_entry_touch(ea_block_cache, ce);
> 2057 mb_cache_entry_put(ea_block_cache, ce);
> 2058 ce = NULL;
> 2059 } else if (bs->bh && s->base == bs->bh->b_data) {
> 2060 /* We were modifying this block in-place. */
> 2061 ea_bdebug(bs->bh, "keeping this block");
> 2062 ext4_xattr_block_cache_insert(ea_block_cache, bs->bh);
> 2063 new_bh = bs->bh;
> 2064 get_bh(new_bh);
> 2065 } else {
> 2066 /* We need to allocate a new block */
> 2067 ext4_fsblk_t goal, block;
> 2068
> 2069 WARN_ON_ONCE(dquot_initialize_needed(inode));
> 2070
> 2071 goal = ext4_group_first_block_no(sb,
> 2072 EXT4_I(inode)->i_block_group);
> 2073
> 2074 /* non-extent files can't have physical blocks past 2^32 */
> 2075 if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
> 2076 goal = goal & EXT4_MAX_BLOCK_FILE_PHYS;
> 2077
> 2078 block = ext4_new_meta_blocks(handle, inode, goal, 0,
> 2079 NULL, &error);
> 2080 if (error)
> 2081 goto cleanup;
> 2082
> 2083 if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
> 2084 BUG_ON(block > EXT4_MAX_BLOCK_FILE_PHYS);
> 2085
> 2086 ea_idebug(inode, "creating block %llu",
> 2087 (unsigned long long)block);
> 2088
> 2089 new_bh = sb_getblk(sb, block);
> 2090 if (unlikely(!new_bh)) {
> 2091 error = -ENOMEM;
> 2092 getblk_failed:
> 2093 ext4_free_blocks(handle, inode, NULL, block, 1,
> 2094 EXT4_FREE_BLOCKS_METADATA);
> 2095 goto cleanup;
> 2096 }
> 2097 error = ext4_xattr_inode_inc_ref_all(handle, inode,
> 2098 ENTRY(header(s->base)+1));
> 2099 if (error)
> 2100 goto getblk_failed;
> 2101 if (ea_inode) {
> 2102 /* Drop the extra ref on ea_inode. */
> 2103 error = ext4_xattr_inode_dec_ref(handle,
> 2104 ea_inode);
> 2105 if (error)
> 2106 ext4_warning_inode(ea_inode,
> 2107 "dec ref error=%d",
> 2108 error);
> 2109 iput(ea_inode);
> 2110 ea_inode = NULL;
> 2111 }
> 2112
> 2113 lock_buffer(new_bh);
> 2114 error = ext4_journal_get_create_access(handle, sb,
> 2115 new_bh, EXT4_JTR_NONE);
> 2116 if (error) {
> 2117 unlock_buffer(new_bh);
> 2118 error = -EIO;
> 2119 goto getblk_failed;
> 2120 }
> 2121 memcpy(new_bh->b_data, s->base, new_bh->b_size);
> 2122 ext4_xattr_block_csum_set(inode, new_bh);
> 2123 set_buffer_uptodate(new_bh);
> 2124 unlock_buffer(new_bh);
> 2125 ext4_xattr_block_cache_insert(ea_block_cache, new_bh);
> 2126 error = ext4_handle_dirty_metadata(handle, inode,
> 2127 new_bh);
> 2128 if (error)
> 2129 goto cleanup;
> 2130 }
> 2131 }
> 2132
> 2133 if (old_ea_inode_quota)
> 2134 ext4_xattr_inode_free_quota(inode, NULL, old_ea_inode_quota);
> 2135
> 2136 /* Update the inode. */
> 2137 EXT4_I(inode)->i_file_acl = new_bh ? new_bh->b_blocknr : 0;
> 2138
> 2139 /* Drop the previous xattr block. */
> 2140 if (bs->bh && bs->bh != new_bh) {
> 2141 struct ext4_xattr_inode_array *ea_inode_array = NULL;
> 2142
> 2143 ext4_xattr_release_block(handle, inode, bs->bh,
> 2144 &ea_inode_array,
> 2145 0 /* extra_credits */);
> 2146 ext4_xattr_inode_array_free(ea_inode_array);
> 2147 }
> 2148 error = 0;
> 2149
> 2150 cleanup:
> 2151 if (ea_inode) {
> 2152 int error2;
> 2153
> 2154 error2 = ext4_xattr_inode_dec_ref(handle, ea_inode);
> 2155 if (error2)
> 2156 ext4_warning_inode(ea_inode, "dec ref error=%d",
> 2157 error2);
> 2158
> 2159 /* If there was an error, revert the quota charge. */
> 2160 if (error)
> 2161 ext4_xattr_inode_free_quota(inode, ea_inode,
> 2162 i_size_read(ea_inode));
> 2163 iput(ea_inode);
> 2164 }
> 2165 if (ce)
> 2166 mb_cache_entry_put(ea_block_cache, ce);
> 2167 brelse(new_bh);
> 2168 if (!(bs->bh && s->base == bs->bh->b_data))
> 2169 kfree(s->base);
> 2170
> 2171 return error;
> 2172
> 2173 cleanup_dquot:
> 2174 dquot_free_block(inode, EXT4_C2B(EXT4_SB(sb), 1));
> 2175 goto cleanup;
> 2176
> 2177 bad_block:
> 2178 EXT4_ERROR_INODE(inode, "bad block %llu",
> 2179 EXT4_I(inode)->i_file_acl);
> 2180 goto cleanup;
> 2181
> 2182 #undef header
> 2183 }
>
> regards,
> dan carpenter
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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