Various notifications of type "BUG kmalloc-4096 () : Redzone overwritten" have been observed recently in various parts of the kernel. After some time, it has been made a relation with the use of BTRFS filesystem.
[ 22.809700] BUG kmalloc-4096 (Tainted: G W ): Redzone overwritten [ 22.809971] -----------------------------------------------------------------------------
[ 22.810286] INFO: 0xbe1a5921-0xfbfc06cd. First byte 0x0 instead of 0xcc [ 22.810866] INFO: Allocated in __load_free_space_cache+0x588/0x780 [btrfs] age=22 cpu=0 pid=224 [ 22.811193] __slab_alloc.constprop.26+0x44/0x70 [ 22.811345] kmem_cache_alloc_trace+0xf0/0x2ec [ 22.811588] __load_free_space_cache+0x588/0x780 [btrfs] [ 22.811848] load_free_space_cache+0xf4/0x1b0 [btrfs] [ 22.812090] cache_block_group+0x1d0/0x3d0 [btrfs] [ 22.812321] find_free_extent+0x680/0x12a4 [btrfs] [ 22.812549] btrfs_reserve_extent+0xec/0x220 [btrfs] [ 22.812785] btrfs_alloc_tree_block+0x178/0x5f4 [btrfs] [ 22.813032] __btrfs_cow_block+0x150/0x5d4 [btrfs] [ 22.813262] btrfs_cow_block+0x194/0x298 [btrfs] [ 22.813484] commit_cowonly_roots+0x44/0x294 [btrfs] [ 22.813718] btrfs_commit_transaction+0x63c/0xc0c [btrfs] [ 22.813973] close_ctree+0xf8/0x2a4 [btrfs] [ 22.814107] generic_shutdown_super+0x80/0x110 [ 22.814250] kill_anon_super+0x18/0x30 [ 22.814437] btrfs_kill_super+0x18/0x90 [btrfs] [ 22.814590] INFO: Freed in proc_cgroup_show+0xc0/0x248 age=41 cpu=0 pid=83 [ 22.814841] proc_cgroup_show+0xc0/0x248 [ 22.814967] proc_single_show+0x54/0x98 [ 22.815086] seq_read+0x278/0x45c [ 22.815190] __vfs_read+0x28/0x17c [ 22.815289] vfs_read+0xa8/0x14c [ 22.815381] ksys_read+0x50/0x94 [ 22.815475] ret_from_syscall+0x0/0x38
Commit 69d2480456d1 ("btrfs: use copy_page for copying pages instead of memcpy") changed the way bitmap blocks are copied. But allthough bitmaps have the size of a page, they were allocated with kzalloc().
Most of the time, kzalloc() allocates aligned blocks of memory, so copy_page() can be used. But when some debug options like SLAB_DEBUG are activated, kzalloc() may return unaligned pointer.
On powerpc, memcpy(), copy_page() and other copying functions use 'dcbz' instruction which provides an entire zeroed cacheline to avoid memory read when the intention is to overwrite a full line. Functions like memcpy() are writen to care about partial cachelines at the start and end of the destination, but copy_page() assumes it gets pages. As pages are naturally cache aligned, copy_page() doesn't care about partial lines. This means that when copy_page() is called with a misaligned pointer, a few leading bytes are zeroed.
To fix it, allocate bitmaps with get_zeroed_page() instead of kzalloc()
Reported-by: Erhard F. erhard_f@mailbox.org Link: https://bugzilla.kernel.org/show_bug.cgi?id=204371 Fixes: 69d2480456d1 ("btrfs: use copy_page for copying pages instead of memcpy") Cc: stable@vger.kernel.org Signed-off-by: Christophe Leroy christophe.leroy@c-s.fr Tested-by: Erhard F. erhard_f@mailbox.org --- fs/btrfs/free-space-cache.c | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-)
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c index 062be9dde4c6..3229a058e025 100644 --- a/fs/btrfs/free-space-cache.c +++ b/fs/btrfs/free-space-cache.c @@ -764,7 +764,7 @@ static int __load_free_space_cache(struct btrfs_root *root, struct inode *inode, } else { ASSERT(num_bitmaps); num_bitmaps--; - e->bitmap = kzalloc(PAGE_SIZE, GFP_NOFS); + e->bitmap = (void *)get_zeroed_page(GFP_NOFS); if (!e->bitmap) { kmem_cache_free( btrfs_free_space_cachep, e); @@ -1881,7 +1881,7 @@ static void free_bitmap(struct btrfs_free_space_ctl *ctl, struct btrfs_free_space *bitmap_info) { unlink_free_space(ctl, bitmap_info); - kfree(bitmap_info->bitmap); + free_page((unsigned long)bitmap_info->bitmap); kmem_cache_free(btrfs_free_space_cachep, bitmap_info); ctl->total_bitmaps--; ctl->op->recalc_thresholds(ctl); @@ -2135,7 +2135,7 @@ static int insert_into_bitmap(struct btrfs_free_space_ctl *ctl, }
/* allocate the bitmap */ - info->bitmap = kzalloc(PAGE_SIZE, GFP_NOFS); + info->bitmap = (void *)get_zeroed_page(GFP_NOFS); spin_lock(&ctl->tree_lock); if (!info->bitmap) { ret = -ENOMEM; @@ -2146,7 +2146,7 @@ static int insert_into_bitmap(struct btrfs_free_space_ctl *ctl,
out: if (info) { - kfree(info->bitmap); + free_page((unsigned long)info->bitmap); kmem_cache_free(btrfs_free_space_cachep, info); }
@@ -2802,7 +2802,7 @@ u64 btrfs_alloc_from_cluster(struct btrfs_block_group_cache *block_group, if (entry->bytes == 0) { ctl->free_extents--; if (entry->bitmap) { - kfree(entry->bitmap); + free_page((unsigned long)entry->bitmap); ctl->total_bitmaps--; ctl->op->recalc_thresholds(ctl); } @@ -3606,7 +3606,7 @@ int test_add_free_space_entry(struct btrfs_block_group_cache *cache, }
if (!map) { - map = kzalloc(PAGE_SIZE, GFP_NOFS); + map = (void *)get_zeroed_page(GFP_NOFS); if (!map) { kmem_cache_free(btrfs_free_space_cachep, info); return -ENOMEM; @@ -3635,7 +3635,7 @@ int test_add_free_space_entry(struct btrfs_block_group_cache *cache,
if (info) kmem_cache_free(btrfs_free_space_cachep, info); - kfree(map); + free_page((unsigned long)map); return 0; }