[PATCH v3] libfs: fix error cast of negative value in simple_attr_write()

[PATCH v3] libfs: fix error cast of negative value in simple_attr_write()

[Yicong Yang]

The attr->set() receive a value of u64, but simple_strtoll() is used
for doing the conversion. It will lead to the error cast if user inputs
a negative value.

Use kstrtoull() instead of simple_strtoll() to convert a string got
from the user to an unsigned value. The former will return '-EINVAL' if
it gets a negetive value, but the latter can't handle the situation
correctly. Make 'val' unsigned long long as what kstrtoull() takes, this
will eliminate the compile warning on no 64-bit architectures.

Fixes: f7b88631a897 ("fs/libfs.c: fix simple_attr_write() on 32bit machines")
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
---
Change since v1:
- address the compile warning for non-64 bit platform.
Change since v2:
Link: https://lore.kernel.org/linux-fsdevel/1605000324-7428-1-git-send-email-yangyicong@hisilicon.com/
- make 'val' unsigned long long and mentioned in the commit
Link: https://lore.kernel.org/linux-fsdevel/1605261369-551-1-git-send-email-yangyicong@hisilicon.com/

 fs/libfs.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index fc34361..7124c2e 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -959,7 +959,7 @@ ssize_t simple_attr_write(struct file *file, const char __user *buf,
 			  size_t len, loff_t *ppos)
 {
 	struct simple_attr *attr;
-	u64 val;
+	unsigned long long val;
 	size_t size;
 	ssize_t ret;
 
@@ -977,7 +977,9 @@ ssize_t simple_attr_write(struct file *file, const char __user *buf,
 		goto out;
 
 	attr->set_buf[size] = '\0';
-	val = simple_strtoll(attr->set_buf, NULL, 0);
+	ret = kstrtoull(attr->set_buf, 0, &val);
+	if (ret)
+		goto out;
 	ret = attr->set(attr->data, val);
 	if (ret == 0)
 		ret = len; /* on success, claim we got the whole input */
-- 
2.8.1

Re: [PATCH v3] libfs: fix error cast of negative value in simple_attr_write()

[Luck, Tony]

On Sat, Nov 14, 2020 at 04:09:16PM +0800, Yicong Yang wrote:
> The attr->set() receive a value of u64, but simple_strtoll() is used
> for doing the conversion. It will lead to the error cast if user inputs
> a negative value.
> 
> Use kstrtoull() instead of simple_strtoll() to convert a string got
> from the user to an unsigned value. The former will return '-EINVAL' if
> it gets a negetive value, but the latter can't handle the situation
> correctly. Make 'val' unsigned long long as what kstrtoull() takes, this
> will eliminate the compile warning on no 64-bit architectures.
> 
> Fixes: f7b88631a897 ("fs/libfs.c: fix simple_attr_write() on 32bit machines")
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> ---
> Change since v1:
> - address the compile warning for non-64 bit platform.
> Change since v2:
> Link: https://lore.kernel.org/linux-fsdevel/1605000324-7428-1-git-send-email-yangyicong@hisilicon.com/
> - make 'val' unsigned long long and mentioned in the commit
> Link: https://lore.kernel.org/linux-fsdevel/1605261369-551-1-git-send-email-yangyicong@hisilicon.com/

Belated error report on this. Some validation team just moved to
v5.10 and found their error injection scripts no longer work.

They have been using:

# echo $((-1 << 12)) > /sys/kernel/debug/apei/einj/param2

to write the mask value 0xfffffffffffff000 for many years ... but
now writing a negative value (-4096) to this file gives an EINVAL error.

Maybe they've been taking advantage of a bug all this time? The
comment for debugfs_create_x64() says it is for reading/writing
an unsigned value. But when a bug fix breaks user code ... then
we are supposed to ask whether that bug is actually a feature.

If there was a debugfs_create_s64() I might just fix einj.c to
use that ... but there isn't :-(

-Tony

Re: [PATCH v3] libfs: fix error cast of negative value in simple_attr_write()

[Yicong Yang]

On 2021/6/4 0:09, Luck, Tony wrote:
> On Sat, Nov 14, 2020 at 04:09:16PM +0800, Yicong Yang wrote:
>> The attr->set() receive a value of u64, but simple_strtoll() is used
>> for doing the conversion. It will lead to the error cast if user inputs
>> a negative value.
>>
>> Use kstrtoull() instead of simple_strtoll() to convert a string got
>> from the user to an unsigned value. The former will return '-EINVAL' if
>> it gets a negetive value, but the latter can't handle the situation
>> correctly. Make 'val' unsigned long long as what kstrtoull() takes, this
>> will eliminate the compile warning on no 64-bit architectures.
>>
>> Fixes: f7b88631a897 ("fs/libfs.c: fix simple_attr_write() on 32bit machines")
>> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
>> ---
>> Change since v1:
>> - address the compile warning for non-64 bit platform.
>> Change since v2:
>> Link: https://lore.kernel.org/linux-fsdevel/1605000324-7428-1-git-send-email-yangyicong@hisilicon.com/
>> - make 'val' unsigned long long and mentioned in the commit
>> Link: https://lore.kernel.org/linux-fsdevel/1605261369-551-1-git-send-email-yangyicong@hisilicon.com/
> 
> Belated error report on this. Some validation team just moved to
> v5.10 and found their error injection scripts no longer work.
> 
> They have been using:
> 
> # echo $((-1 << 12)) > /sys/kernel/debug/apei/einj/param2
> 
> to write the mask value 0xfffffffffffff000 for many years ... but
> now writing a negative value (-4096) to this file gives an EINVAL error.
> 

ok. i didn't know this usage. I was using debugfs for my driver but
found I cannot figure out whether user has entered a negative value.

> Maybe they've been taking advantage of a bug all this time? The
> comment for debugfs_create_x64() says it is for reading/writing
> an unsigned value. But when a bug fix breaks user code ... then
> we are supposed to ask whether that bug is actually a feature.
> 

ok. sounds reasonable.

> If there was a debugfs_create_s64() I might just fix einj.c to
> use that ... but there isn't :-(

yes, a debugfs_create_s64() seems better for this case.
But it's okay for me to revert this fix if we regard this bug as
a feature.

Thanks
Yicong