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Subject: Re: [PATCH v5] f2fs: fix to avoid accessing xattr across the boundary
To:     Jaegeuk Kim <jaegeuk@kernel.org>,
        Randall Huang <huangrandall@google.com>
CC:     <linux-f2fs-devel@lists.sourceforge.net>,
        <linux-kernel@vger.kernel.org>
References: <20190411082646.169977-1-huangrandall@google.com>
 <20190509041535.GA62877@jaegeuk-macbookpro.roam.corp.google.com>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <bdba5d01-4a31-d8f2-4805-81d167047c84@huawei.com>
Date:   Thu, 9 May 2019 15:07:18 +0800
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On 2019/5/9 12:15, Jaegeuk Kim wrote:
> On 04/11, 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()
>>
>> v4:
>> * change boundry definition
>>
>> v5:
>> * revise boundry definition
>> ---
>>  fs/f2fs/xattr.c | 32 +++++++++++++++++++++++++++-----
>>  1 file changed, 27 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/f2fs/xattr.c b/fs/f2fs/xattr.c
>> index 848a785abe25..587429e29a69 100644
>> --- a/fs/f2fs/xattr.c
>> +++ b/fs/f2fs/xattr.c
>> @@ -202,12 +202,18 @@ 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,
>> +				void *last_base_addr, int index,
>> +				size_t len, const char *name)
>>  {
>>  	struct f2fs_xattr_entry *entry;
>>  
>>  	list_for_each_xattr(entry, base_addr) {
>> +		if ((void *)(entry) + sizeof(__u32) > last_base_addr ||
>> +			(void *)XATTR_NEXT_ENTRY(entry) +
>> +			sizeof(__u32) > last_base_addr)
>> +			return NULL;
>> +
>>  		if (entry->e_name_index != index)
>>  			continue;
>>  		if (entry->e_name_len != len)
>> @@ -298,6 +304,7 @@ static int lookup_all_xattrs(struct inode *inode, struct page *ipage,
>>  				void **base_addr, int *base_size)
>>  {
>>  	void *cur_addr, *txattr_addr, *last_addr = NULL;
>> +	void *last_txattr_addr = NULL;
>>  	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);
>> @@ -311,6 +318,8 @@ static int lookup_all_xattrs(struct inode *inode, struct page *ipage,
>>  	if (!txattr_addr)
>>  		return -ENOMEM;
>>  
>> +	last_txattr_addr = (void *)txattr_addr + inline_size + size;
> 
> I just found this should be + XATTR_PADDING_SIZE. Otherwise, generic/026 fails.
> Let me know, if there is any other concern below.

We're trying to use [txattr_addr, last_txattr_addr] to indicate valid range of
xattr datas, any valid entries across the boundary is not allowed, so I think
it's correct to exclude padding space for last_txattr_addr.

I added log and tested generic/026, found that:

[365804.868431] base:ffff89e391f8e000, last:ffff89e391f8f0b0
[365804.868433] entry:ffff89e391f8e018, next:ffff89e391f8f0b0

The root cause of this issue is caused by below condition:

		(void *)XATTR_NEXT_ENTRY(entry) +
		sizeof(__u32) > last_base_addr)

It occurs when the end address of last valid entry is located in range
(last_base_addr - XATTR_PADDING_SIZE, last_base_addr], then next invalid entry's
head will across last_base_addr, result in returning -EFAULT.

So anyway, please try below diff instead. ;)

diff --git a/fs/f2fs/xattr.c b/fs/f2fs/xattr.c
index 019778fb9a0d..217bf7c2bb7a 100644
--- a/fs/f2fs/xattr.c
+++ b/fs/f2fs/xattr.c
@@ -210,10 +210,8 @@ static struct f2fs_xattr_entry *__find_xattr(void *base_addr,

 	list_for_each_xattr(entry, base_addr) {
 		if ((void *)(entry) + sizeof(__u32) > last_base_addr ||
-			(void *)XATTR_NEXT_ENTRY(entry) +
-			sizeof(__u32) > last_base_addr)
+			(void *)XATTR_NEXT_ENTRY(entry) > last_base_addr)
 			return NULL;

 		if (entry->e_name_index != index)

Thanks,

> 
> ---
>  fs/f2fs/xattr.c | 12 ++++--------
>  fs/f2fs/xattr.h |  2 ++
>  2 files changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/f2fs/xattr.c b/fs/f2fs/xattr.c
> index 019778fb9a0d..cd199a09d436 100644
> --- a/fs/f2fs/xattr.c
> +++ b/fs/f2fs/xattr.c
> @@ -306,19 +306,18 @@ static int lookup_all_xattrs(struct inode *inode, struct page *ipage,
>  	void *cur_addr, *txattr_addr, *last_addr = NULL;
>  	void *last_txattr_addr = NULL;
>  	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);
>  	int err = 0;
>  
> -	if (!size && !inline_size)
> +	if (!xnid && !inline_size)
>  		return -ENODATA;
>  
> -	*base_size = inline_size + size + XATTR_PADDING_SIZE;
> +	*base_size = XATTR_SIZE(xnid, inode);
>  	txattr_addr = f2fs_kzalloc(F2FS_I_SB(inode), *base_size, GFP_NOFS);
>  	if (!txattr_addr)
>  		return -ENOMEM;
>  
> -	last_txattr_addr = (void *)txattr_addr + inline_size + size;
> +	last_txattr_addr = (void *)txattr_addr + *base_size;
>  
>  	/* read from inline xattr */
>  	if (inline_size) {
> @@ -599,9 +598,6 @@ static int __f2fs_setxattr(struct inode *inode, int index,
>  	size_t len;
>  	__u32 new_hsize;
>  	nid_t xnid = F2FS_I(inode)->i_xattr_nid;
> -	unsigned int xnode_size = xnid ? VALID_XATTR_BLOCK_SIZE : 0;
> -	unsigned int inline_size = inline_xattr_size(inode);
> -
>  	int error = 0;
>  
>  	if (name == NULL)
> @@ -621,7 +617,7 @@ static int __f2fs_setxattr(struct inode *inode, int index,
>  	error = read_all_xattrs(inode, ipage, &base_addr);
>  	if (error)
>  		return error;
> -	last_base_addr = (void *)base_addr + xnode_size + inline_size;
> +	last_base_addr = (void *)base_addr + XATTR_SIZE(xnid, inode);
>  
>  	/* find entry with wanted name. */
>  	here = __find_xattr(base_addr, last_base_addr, index, len, name);
> diff --git a/fs/f2fs/xattr.h b/fs/f2fs/xattr.h
> index 9172ee082ca8..1eca1a2d996a 100644
> --- a/fs/f2fs/xattr.h
> +++ b/fs/f2fs/xattr.h
> @@ -71,6 +71,8 @@ struct f2fs_xattr_entry {
>  				entry = XATTR_NEXT_ENTRY(entry))
>  #define VALID_XATTR_BLOCK_SIZE	(PAGE_SIZE - sizeof(struct node_footer))
>  #define XATTR_PADDING_SIZE	(sizeof(__u32))
> +#define XATTR_SIZE(x,i)		(((x) ? VALID_XATTR_BLOCK_SIZE : 0) +	\
> +				(inline_xattr_size(i)) + XATTR_PADDING_SIZE)
>  #define MIN_OFFSET(i)		XATTR_ALIGN(inline_xattr_size(i) +	\
>  						VALID_XATTR_BLOCK_SIZE)
>  
>