Re: [PATCH 2/3] hfs: fix high memory mapping in hfs_bnode_read
From: Desmond Cheong Zhi Xi
Date: Tue Jun 29 2021 - 22:36:16 EST
On 30/6/21 3:31 am, Viacheslav Dubeyko wrote:
On Jun 29, 2021, at 7:48 AM, Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx> wrote:
Pages that we read in hfs_bnode_read need to be kmapped into kernel
address space. However, currently only the 0th page is kmapped. If the
given offset + length exceeds this 0th page, then we have an invalid
memory access.
To fix this, we use the same logic used in hfsplus' version of
hfs_bnode_read to kmap each page of relevant data that we copy.
An example of invalid memory access occurring without this fix can be
seen in the following crash report:
==================================================================
BUG: KASAN: use-after-free in memcpy include/linux/fortify-string.h:191 [inline]
BUG: KASAN: use-after-free in hfs_bnode_read+0xc4/0xe0 fs/hfs/bnode.c:26
Read of size 2 at addr ffff888125fdcffe by task syz-executor5/4634
CPU: 0 PID: 4634 Comm: syz-executor5 Not tainted 5.13.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:79 [inline]
dump_stack+0x195/0x1f8 lib/dump_stack.c:120
print_address_description.constprop.0+0x1d/0x110 mm/kasan/report.c:233
__kasan_report mm/kasan/report.c:419 [inline]
kasan_report.cold+0x7b/0xd4 mm/kasan/report.c:436
check_region_inline mm/kasan/generic.c:180 [inline]
kasan_check_range+0x154/0x1b0 mm/kasan/generic.c:186
memcpy+0x24/0x60 mm/kasan/shadow.c:65
memcpy include/linux/fortify-string.h:191 [inline]
hfs_bnode_read+0xc4/0xe0 fs/hfs/bnode.c:26
hfs_bnode_read_u16 fs/hfs/bnode.c:34 [inline]
hfs_bnode_find+0x880/0xcc0 fs/hfs/bnode.c:365
hfs_brec_find+0x2d8/0x540 fs/hfs/bfind.c:126
hfs_brec_read+0x27/0x120 fs/hfs/bfind.c:165
hfs_cat_find_brec+0x19a/0x3b0 fs/hfs/catalog.c:194
hfs_fill_super+0xc13/0x1460 fs/hfs/super.c:419
mount_bdev+0x331/0x3f0 fs/super.c:1368
hfs_mount+0x35/0x40 fs/hfs/super.c:457
legacy_get_tree+0x10c/0x220 fs/fs_context.c:592
vfs_get_tree+0x93/0x300 fs/super.c:1498
do_new_mount fs/namespace.c:2905 [inline]
path_mount+0x13f5/0x20e0 fs/namespace.c:3235
do_mount fs/namespace.c:3248 [inline]
__do_sys_mount fs/namespace.c:3456 [inline]
__se_sys_mount fs/namespace.c:3433 [inline]
__x64_sys_mount+0x2b8/0x340 fs/namespace.c:3433
do_syscall_64+0x37/0xc0 arch/x86/entry/common.c:47
entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x45e63a
Code: 48 c7 c2 bc ff ff ff f7 d8 64 89 02 b8 ff ff ff ff eb d2 e8 88 04 00 00 0f 1f 84 00 00 00 00 00 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f9404d410d8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000020000248 RCX: 000000000045e63a
RDX: 0000000020000000 RSI: 0000000020000100 RDI: 00007f9404d41120
RBP: 00007f9404d41120 R08: 00000000200002c0 R09: 0000000020000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003
R13: 0000000000000003 R14: 00000000004ad5d8 R15: 0000000000000000
The buggy address belongs to the page:
page:00000000dadbcf3e refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x125fdc
flags: 0x2fffc0000000000(node=0|zone=2|lastcpupid=0x3fff)
raw: 02fffc0000000000 ffffea000497f748 ffffea000497f6c8 0000000000000000
raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected
Memory state around the buggy address:
ffff888125fdce80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888125fdcf00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888125fdcf80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
^
ffff888125fdd000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
ffff888125fdd080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
---
fs/hfs/bnode.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)
diff --git a/fs/hfs/bnode.c b/fs/hfs/bnode.c
index b63a4df7327b..936cfa763224 100644
--- a/fs/hfs/bnode.c
+++ b/fs/hfs/bnode.c
@@ -18,13 +18,23 @@
void hfs_bnode_read(struct hfs_bnode *node, void *buf,
int off, int len)
{
- struct page *page;
+ struct page **pagep;
+ int l;
off += node->page_offset;
- page = node->page[0];
+ pagep = node->page + (off >> PAGE_SHIFT);
I would like to have a check here that we are not out of the page array. Could you add this check?
Also, maybe, node->page[index] could look much safer here. What do you think?
Hi Slava,
Thanks for the review.
Checking that we don't exceed the page array sounds good. Also agree
that node->page[index] looks more readable.
+ off &= ~PAGE_MASK;
- memcpy(buf, kmap(page) + off, len);
- kunmap(page);
+ l = min_t(int, len, PAGE_SIZE - off);
Maybe, it makes sense to use more informative name of the variable instead of “l”?
+ memcpy(buf, kmap(*pagep) + off, l);
I suppose that it could be good to have a check that we do not overflow the buffer. How do you feel about it?
The computation of `l = min_t(int, len, PAGE_SIZE - off);` ensures that
we don't overflow the buffer because the number of bytes read won't
exceed the given len. But I think you're right that this can be made
more explicit, alongside giving "l" a more informative name.
+ kunmap(*pagep);
What’s about kmap_atomic/kunmap_atomic in this function?
Thanks,
Slava.
kmap_atomic/kunmap_atomic sounds good to me. I'll followup with a v2
series. Thanks again!
Best wishes,
Desmond
+
+ while ((len -= l) != 0) {
+ buf += l;
+ l = min_t(int, len, PAGE_SIZE);
+ memcpy(buf, kmap(*++pagep), l);
+ kunmap(*pagep);
+ }
}
u16 hfs_bnode_read_u16(struct hfs_bnode *node, int off)
--
2.25.1