linux-ext4 - [PATCH 2/2] e2fsck: fix use after free in calculate

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Date:   Tue, 26 Nov 2019 18:03:59 +0900
From:   Wang Shilong <wangshilong1991@...il.com>
To:     linux-ext4@...r.kernel.org
Cc:     adilger@...ger.ca, lixi@....com, wshilong@....com
Subject: [PATCH 2/2] e2fsck: fix use after free in calculate_tree()

From: Wang Shilong <wshilong@....com>

Hit following Seg errors randomly when running f_large_dir test:

+Signal (11) SIGSEGV si_code=SEGV_MAPERR fault addr=0x7f02cfffbc1a
+../e2fsck/e2fsck[0x43766e]
+/lib64/libpthread.so.0(+0xf7e0)[0x7f02d8c9a7e0]
+../e2fsck/e2fsck(e2fsck_rehash_dir+0x10f3)[0x436173]
+../e2fsck/e2fsck(e2fsck_rehash_directories+0xf4)[0x4362d4]
+../e2fsck/e2fsck(e2fsck_pass3+0x722)[0x4292c2]
+../e2fsck/e2fsck(e2fsck_run+0x47)[0x414ef7]
+../e2fsck/e2fsck(main+0x1c1d)[0x41319d]
+/lib64/libc.so.6(__libc_start_main+0x100)[0x7f02d8915d20]
+../e2fsck/e2fsck[0x40fc59]
+Exit status is 8

gdb output is:
0x436173 is in e2fsck_rehash_dir (rehash.c:752).
warning: Source file is more recent than executable.
747					dx_ent->hash =
748						ext2fs_cpu_to_le32(outdir->hashes[i]);
749				dx_ent++;
750				c3--;
751			}
752			int_limit->count = ext2fs_cpu_to_le16(limit->limit - c2);
753			int_limit->limit = ext2fs_cpu_to_le16(limit->limit);
754
755			limit->count = ext2fs_cpu_to_le16(limit->limit - c3);
756			limit->limit = ext2fs_cpu_to_le16(limit->limit);

The problem is alloc_blocks() will call get_next_block()
which might reallocate @outdir->buf, and memory address
could be changed after this. @int_limit and @root should
be recalculated based on new start address. Otherwise,
it will try to access freed memory and cause SEGV_MAPERR
errors.

Signed-off-by: Wang Shilong <wshilong@....com>
---
 e2fsck/rehash.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/e2fsck/rehash.c b/e2fsck/rehash.c
index 5250652e..0eb99328 100644
--- a/e2fsck/rehash.c
+++ b/e2fsck/rehash.c
@@ -636,6 +636,9 @@ static int alloc_blocks(ext2_filsys fs,
 	if (retval)
 		return retval;
 
+	/* outdir->buf might be reallocated */
+	*prev_ent = (struct ext2_dx_entry *) (outdir->buf + *prev_offset);
+
 	*next_ent = set_int_node(fs, block_start);
 	*limit = (struct ext2_dx_countlimit *)(*next_ent);
 	if (next_offset)
@@ -725,12 +728,18 @@ static errcode_t calculate_tree(ext2_filsys fs,
 					return retval;
 			}
 			if (c3 == 0) {
+				int delta1 = int_offset;;
+				int delta2 = (char *)root - outdir->buf;
+
 				retval = alloc_blocks(fs, &limit, &int_ent,
 						      &dx_ent, &int_offset,
 						      NULL, outdir, i, &c2,
 						      &c3);
 				if (retval)
 					return retval;
+				/* outdir->buf might be reallocated */
+				int_limit = (struct ext2_dx_countlimit *)(outdir->buf + delta1);
+				root = (struct ext2_dx_entry *)(outdir->buf + delta2);
 
 			}
 			dx_ent->block = ext2fs_cpu_to_le32(i);
-- 
2.21.0