staging: erofs: fix illegal address access under memory pressure

commit 1e5ceeab6929585512c63d05911d6657064abf7b upstream.

Considering a read request with two decompressed file pages,
If a decompression work cannot be started on the previous page
due to memory pressure but in-memory LTP map lookup is done,
builder->work should be still NULL.

Moreover, if the current page also belongs to the same map,
it won't try to start the decompression work again and then
run into trouble.

This patch aims to solve the above issue only with little changes
as much as possible in order to make the fix backport easier.

kernel message is:
<4>[1051408.015930s]SLUB: Unable to allocate memory on node -1, gfp=0x2408040(GFP_NOFS|__GFP_ZERO)
<4>[1051408.015930s]  cache: erofs_compress, object size: 144, buffer size: 144, default order: 0, min order: 0
<4>[1051408.015930s]  node 0: slabs: 98, objs: 2744, free: 0
  * Cannot allocate the decompression work

<3>[1051408.015960s]erofs: z_erofs_vle_normalaccess_readpages, readahead error at page 1008 of nid 5391488
  * Note that the previous page was failed to read

<0>[1051408.015960s]Internal error: Accessing user space memory outside uaccess.h routines: 96000005 [kutemeikito#1] PREEMPT SMP
...
<4>[1051408.015991s]Hardware name: kirin710 (DT)
...
<4>[1051408.016021s]PC is at z_erofs_vle_work_add_page+0xa0/0x17c
<4>[1051408.016021s]LR is at z_erofs_do_read_page+0x12c/0xcf0
...
<4>[1051408.018096s][<ffffff80c6fb0fd4>] z_erofs_vle_work_add_page+0xa0/0x17c
<4>[1051408.018096s][<ffffff80c6fb3814>] z_erofs_vle_normalaccess_readpages+0x1a0/0x37c
<4>[1051408.018096s][<ffffff80c6d670b8>] read_pages+0x70/0x190
<4>[1051408.018127s][<ffffff80c6d6736c>] __do_page_cache_readahead+0x194/0x1a8
<4>[1051408.018127s][<ffffff80c6d59318>] filemap_fault+0x398/0x684
<4>[1051408.018127s][<ffffff80c6d8a9e0>] __do_fault+0x8c/0x138
<4>[1051408.018127s][<ffffff80c6d8f90c>] handle_pte_fault+0x730/0xb7c
<4>[1051408.018127s][<ffffff80c6d8fe04>] __handle_mm_fault+0xac/0xf4
<4>[1051408.018157s][<ffffff80c6d8fec8>] handle_mm_fault+0x7c/0x118
<4>[1051408.018157s][<ffffff80c8c52998>] do_page_fault+0x354/0x474
<4>[1051408.018157s][<ffffff80c8c52af8>] do_translation_fault+0x40/0x48
<4>[1051408.018157s][<ffffff80c6c002f4>] do_mem_abort+0x80/0x100
<4>[1051408.018310s]---[ end trace 9f4009a3283bd78b ]---

Fixes: 3883a79abd02 ("staging: erofs: introduce VLE decompression support")
Cc: <[email protected]> # 4.19+
Signed-off-by: Gao Xiang <[email protected]>
Reviewed-by: Chao Yu <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Signed-off-by: Cyber Knight <[email protected]>
Signed-off-by: Edwiin Kusuma Jaya <[email protected]>

drivers/staging/erofs/unzip_vle.c

@@ -622,8 +622,12 @@ static int z_erofs_do_read_page(struct z_erofs_vle_frontend *fe,
 
 	/* lucky, within the range of the current map_blocks */
 	if (offset + cur >= map->m_la &&
-		offset + cur < map->m_la + map->m_llen)
+		offset + cur < map->m_la + map->m_llen) {
+		/* didn't get a valid unzip work previously (very rare) */
+		if (!builder->work)
+			goto restart_now;
 		goto hitted;
+	}
 
 	/* go ahead the next map_blocks */
 	debugln("%s: [out-of-range] pos %llu", __func__, offset + cur);
@@ -637,6 +641,7 @@ static int z_erofs_do_read_page(struct z_erofs_vle_frontend *fe,
 	if (unlikely(err))
 		goto err_out;
 
+restart_now:
 	if (unlikely(!(map->m_flags & EROFS_MAP_MAPPED)))
 		goto hitted;