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SubjectRe: [PATCH 2/3] hfs: fix high memory mapping in hfs_bnode_read
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> On Jun 29, 2021, at 7:48 AM, Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> 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@gmail.com>
> ---
> 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?

> + 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?

> + kunmap(*pagep);

What’s about kmap_atomic/kunmap_atomic in this function?

Thanks,
Slava.

> +
> + 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
>

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