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erofs: fix buffer copy overflow of ztailpacking feature

I got some KASAN report as below:

[ 46.959738] ==================================================================
[ 46.960430] BUG: KASAN: use-after-free in z_erofs_shifted_transform+0x2bd/0x370
[ 46.960430] Read of size 4074 at addr ffff8880300c2f8e by task fssum/188
...
[ 46.960430] Call Trace:
[ 46.960430] <TASK>
[ 46.960430] dump_stack_lvl+0x41/0x5e
[ 46.960430] print_report.cold+0xb2/0x6b7
[ 46.960430] ? z_erofs_shifted_transform+0x2bd/0x370
[ 46.960430] kasan_report+0x8a/0x140
[ 46.960430] ? z_erofs_shifted_transform+0x2bd/0x370
[ 46.960430] kasan_check_range+0x14d/0x1d0
[ 46.960430] memcpy+0x20/0x60
[ 46.960430] z_erofs_shifted_transform+0x2bd/0x370
[ 46.960430] z_erofs_decompress_pcluster+0xaae/0x1080

The root cause is that the tail pcluster won't be a complete filesystem
block anymore. So if ztailpacking is used, the second part of an
uncompressed tail pcluster may not be ``rq->pageofs_out``.

Fixes: ab749badf9f4 ("erofs: support unaligned data decompression")
Fixes: cecf864d3d76 ("erofs: support inline data decompression")
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220512115833.24175-1-hsiangkao@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Gao Xiang dcbe6803 2833f4bb

+3 -2
+3 -2
fs/erofs/decompressor.c
··· 320 320 PAGE_ALIGN(rq->pageofs_out + rq->outputsize) >> PAGE_SHIFT; 321 321 const unsigned int righthalf = min_t(unsigned int, rq->outputsize, 322 322 PAGE_SIZE - rq->pageofs_out); 323 + const unsigned int lefthalf = rq->outputsize - righthalf; 323 324 unsigned char *src, *dst; 324 325 325 326 if (nrpages_out > 2) { ··· 343 342 if (nrpages_out == 2) { 344 343 DBG_BUGON(!rq->out[1]); 345 344 if (rq->out[1] == *rq->in) { 346 - memmove(src, src + righthalf, rq->pageofs_out); 345 + memmove(src, src + righthalf, lefthalf); 347 346 } else { 348 347 dst = kmap_atomic(rq->out[1]); 349 - memcpy(dst, src + righthalf, rq->pageofs_out); 348 + memcpy(dst, src + righthalf, lefthalf); 350 349 kunmap_atomic(dst); 351 350 } 352 351 }