Re: [PATCH] f2fs: fix to avoid accessing xattr across the boundary

[Chao Yu]

Hi Randall,

On 2019/4/8 16:50, Randall Huang wrote:
> When we traverse xattr entries via __find_xattr(),
> if the raw filesystem content is faked or any hardware failure occurs,
> out-of-bound error can be detected by KASAN.
> Fix the issue by introducing boundary check.
> 
> [   38.402878] c7   1827 BUG: KASAN: slab-out-of-bounds in f2fs_getxattr+0x518/0x68c
> [   38.402891] c7   1827 Read of size 4 at addr ffffffc0b6fb35dc by task
> [   38.402935] c7   1827 Call trace:
> [   38.402952] c7   1827 [<ffffff900809003c>] dump_backtrace+0x0/0x6bc
> [   38.402966] c7   1827 [<ffffff9008090030>] show_stack+0x20/0x2c
> [   38.402981] c7   1827 [<ffffff900871ab10>] dump_stack+0xfc/0x140
> [   38.402995] c7   1827 [<ffffff9008325c40>] print_address_description+0x80/0x2d8
> [   38.403009] c7   1827 [<ffffff900832629c>] kasan_report_error+0x198/0x1fc
> [   38.403022] c7   1827 [<ffffff9008326104>] kasan_report_error+0x0/0x1fc
> [   38.403037] c7   1827 [<ffffff9008325000>] __asan_load4+0x1b0/0x1b8
> [   38.403051] c7   1827 [<ffffff90085fcc44>] f2fs_getxattr+0x518/0x68c
> [   38.403066] c7   1827 [<ffffff90085fc508>] f2fs_xattr_generic_get+0xb0/0xd0
> [   38.403080] c7   1827 [<ffffff9008395708>] __vfs_getxattr+0x1f4/0x1fc
> [   38.403096] c7   1827 [<ffffff9008621bd0>] inode_doinit_with_dentry+0x360/0x938
> [   38.403109] c7   1827 [<ffffff900862d6cc>] selinux_d_instantiate+0x2c/0x38
> [   38.403123] c7   1827 [<ffffff900861b018>] security_d_instantiate+0x68/0x98
> [   38.403136] c7   1827 [<ffffff9008377db8>] d_splice_alias+0x58/0x348
> [   38.403149] c7   1827 [<ffffff900858d16c>] f2fs_lookup+0x608/0x774
> [   38.403163] c7   1827 [<ffffff900835eacc>] lookup_slow+0x1e0/0x2cc
> [   38.403177] c7   1827 [<ffffff9008367fe0>] walk_component+0x160/0x520
> [   38.403190] c7   1827 [<ffffff9008369ef4>] path_lookupat+0x110/0x2b4
> [   38.403203] c7   1827 [<ffffff900835dd38>] filename_lookup+0x1d8/0x3a8
> [   38.403216] c7   1827 [<ffffff900835eeb0>] user_path_at_empty+0x54/0x68
> [   38.403229] c7   1827 [<ffffff9008395f44>] SyS_getxattr+0xb4/0x18c
> [   38.403241] c7   1827 [<ffffff9008084200>] el0_svc_naked+0x34/0x38
> 
> Bug: 126558260
> 
> Signed-off-by: Randall Huang <huangrandall@xxxxxxxxxx>
> ---
>  fs/f2fs/xattr.c | 22 ++++++++++++++++------
>  1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/f2fs/xattr.c b/fs/f2fs/xattr.c
> index 848a785abe25..0531c1e38275 100644
> --- a/fs/f2fs/xattr.c
> +++ b/fs/f2fs/xattr.c
> @@ -202,12 +202,17 @@ static inline const struct xattr_handler *f2fs_xattr_handler(int index)
>  	return handler;
>  }
>  
> -static struct f2fs_xattr_entry *__find_xattr(void *base_addr, int index,
> -					size_t len, const char *name)
> +static struct f2fs_xattr_entry *__find_xattr(void *base_addr,
> +				int base_addr_limit, int index,

unsigned int max_size,

> +				size_t len, const char *name)
>  {
>  	struct f2fs_xattr_entry *entry;
> +	void *max_addr = base_addr + ENTRY_SIZE(XATTR_ENTRY(base_addr)) +
> +		base_addr_limit - 1;

If I'm not missing something, shouldn't it be?

void *max_addr = base_addr + max_size;

>  
>  	list_for_each_xattr(entry, base_addr) {
> +		if ((void *)entry + sizeof(__u32) > max_addr)
> +			return NULL;
>  		if (entry->e_name_index != index)
>  			continue;
>  		if (entry->e_name_len != len)
> @@ -337,9 +342,9 @@ static int lookup_all_xattrs(struct inode *inode, struct page *ipage,
>  	else
>  		cur_addr = txattr_addr;
>  
> -	*xe = __find_xattr(cur_addr, index, len, name);
> +	*xe = __find_xattr(cur_addr, XATTR_PADDING_SIZE, index, len, name);

max_size = *base_size - (txattr_addr - cur_addr);
*xe = __find_xattr(cur_addr, max_size, index, len, name);

>  check:
> -	if (IS_XATTR_LAST_ENTRY(*xe)) {
> +	if (!*xe || IS_XATTR_LAST_ENTRY(*xe)) {

If xattr entry across boundary of max xattr space size, maybe we'd better return -EFAULT
which can be distinguished from a real -ENODATA error, latter, we can set SBI_NEED_FSCK
to give a repairing hint to fsck. :)

>  		err = -ENODATA;
>  		goto out;
>  	}
> @@ -606,9 +611,14 @@ static int __f2fs_setxattr(struct inode *inode, int index,
>  		return error;
>  
>  	/* find entry with wanted name. */
> -	here = __find_xattr(base_addr, index, len, name);
> +	here = __find_xattr(base_addr, inline_xattr_size(inode) +
> +			VALID_XATTR_BLOCK_SIZE + XATTR_PADDING_SIZE,
> +			index, len, name);

unsigned int size = F2FS_I(inode)->i_xattr_nid ? VALID_XATTR_BLOCK_SIZE : 0;
unsigned int max_size = inline_xattr_size(inode) + size + XATTR_PADDING_SIZE;

here = __find_xattr(..., max_size, ...);

if (!here)
	return -EFAULT;

Thanks,

>  
> -	found = IS_XATTR_LAST_ENTRY(here) ? 0 : 1;
> +	if (!here)
> +		found = 0;
> +	else
> +		found = IS_XATTR_LAST_ENTRY(here) ? 0 : 1;
>  
>  	if (found) {
>  		if ((flags & XATTR_CREATE)) {
>