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Subject: Re: [PATCH v3] f2fs: fix to avoid accessing xattr across the boundary
To:     Randall Huang <huangrandall@google.com>, <jaegeuk@kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>,
        <linux-kernel@vger.kernel.org>
References: <20190409085308.212682-1-huangrandall@google.com>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <c66b5be1-8bd5-6153-5aa8-959977cacfb0@huawei.com>
Date:   Tue, 9 Apr 2019 18:22:55 +0800
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On 2019/4/9 16:53, 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@google.com>
> ---
> v2:
> * return EFAULT if OOB error is detected
> 
> v3:
> * fix typo in setxattr()
> ---
>  fs/f2fs/xattr.c | 31 +++++++++++++++++++++++++++----
>  1 file changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/f2fs/xattr.c b/fs/f2fs/xattr.c
> index 848a785abe25..381f14b02a78 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,
> +				unsigned int max_size, int index,
> +				size_t len, const char *name)
>  {
>  	struct f2fs_xattr_entry *entry;
> +	void *max_addr = base_addr + ENTRY_SIZE(XATTR_ENTRY(base_addr)) +
> +				max_size - 1;

Hi Randall,

I think this is not right, I enable noinline_xattr mount option, and add
printk to see the status here, it shows

__find_xattr, base_addr:ffff8e709ba92000, max_addr:ffff8e709baa131f, max_size:4

ffff8e709baa131f - ffff8e709ba92000 = F31F

>  
>  	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)
> @@ -301,6 +306,7 @@ static int lookup_all_xattrs(struct inode *inode, struct page *ipage,
>  	nid_t xnid = F2FS_I(inode)->i_xattr_nid;
>  	unsigned int size = xnid ? VALID_XATTR_BLOCK_SIZE : 0;
>  	unsigned int inline_size = inline_xattr_size(inode);
> +	unsigned int max_size = inline_size + size + XATTR_PADDING_SIZE;
>  	int err = 0;
>  
>  	if (!size && !inline_size)
> @@ -323,6 +329,7 @@ static int lookup_all_xattrs(struct inode *inode, struct page *ipage,
>  			*base_size = inline_size;
>  			goto check;
>  		}
> +		max_size -= inline_size;

The cur_addr pointer may point to middle of inline xattr space due to below code? we
should consider such case here.

if (last_addr)
	cur_addr = XATTR_HDR(last_addr) - 1;

>  	}
>  
>  	/* read from xattr node block */
> @@ -330,6 +337,8 @@ static int lookup_all_xattrs(struct inode *inode, struct page *ipage,
>  		err = read_xattr_block(inode, txattr_addr);
>  		if (err)
>  			goto out;
> +
> +		max_size -= size;

We should not shrink max_size after loading xattr block's data.

Thanks,

>  	}
>  
>  	if (last_addr)
> @@ -337,7 +346,12 @@ 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, max_size, index, len, name);
> +	if (!*xe) {
> +		err = -EFAULT;
> +		goto out;
> +	}
> +
>  check:
>  	if (IS_XATTR_LAST_ENTRY(*xe)) {
>  		err = -ENODATA;
> @@ -585,6 +599,11 @@ static int __f2fs_setxattr(struct inode *inode, int index,
>  	int found, newsize;
>  	size_t len;
>  	__u32 new_hsize;
> +	nid_t xnid = F2FS_I(inode)->i_xattr_nid;
> +	unsigned int xattr_nid_size = xnid ? VALID_XATTR_BLOCK_SIZE : 0;
> +	unsigned int inline_size = inline_xattr_size(inode);
> +	unsigned int max_size = inline_size + xattr_nid_size +
> +				 XATTR_PADDING_SIZE;
>  	int error = 0;
>  
>  	if (name == NULL)
> @@ -606,7 +625,11 @@ 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, max_size, index, len, name);
> +	if (!here) {
> +		error = -EFAULT;
> +		goto exit;
> +	}
>  
>  	found = IS_XATTR_LAST_ENTRY(here) ? 0 : 1;
>  
>