[PATCH] libfs: fix error format in simple_attr_write()

[PATCH] libfs: fix error format in simple_attr_write()

[Eliav Farber]

In commit 488dac0c9237 ("libfs: fix error cast of negative value in
simple_attr_write()"), simple_attr_write() was changed to use kstrtoull()
instead of simple_strtoll() to convert a string got from a user.
A user trying to set a negative value will get an error.

This is wrong since it breaks all the places that use
DEFINE_DEBUGFS_ATTRIBUTE() with format of a signed integer.

For the record there are 43 current users of signed integer which are
likely to be effected by this:

$ git grep -n -A1 -w DEFINE_DEBUGFS_ATTRIBUTE | grep ');' |
sed 's,.*\(".*%.*"\).*,\1,' | sort | uniq -c
  1 "%08llx\n"
  5 "0x%016llx\n"
  5 "0x%02llx\n"
  5 "0x%04llx\n"
 13 "0x%08llx\n"
  1 "0x%4.4llx\n"
  3 "0x%.4llx\n"
  4 "0x%llx\n"
  1 "%1lld\n"
 40 "%lld\n"
  2 "%lli\n"
129 "%llu\n"
  1 "%#llx\n"
  2 "%llx\n"

u64 is not an issue for negative numbers.
The %lld and %llu in any case are for 64-bit value, representing it as
unsigned simplifies the generic code, but it doesn't mean we can't keep
their signed value if we know that.

This change uses sscanf() to fix the problem since it does the conversion
based on the supplied format string.

Fixes: 488dac0c9237 ("libfs: fix error cast of negative value in simple_attr_write()")
Signed-off-by: Eliav Farber <farbere@amazon.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@intel.com>
---
 fs/libfs.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index 31b0ddf01c31..4815c6b4fa02 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -1016,9 +1016,12 @@ ssize_t simple_attr_write(struct file *file, const char __user *buf,
 		goto out;
 
 	attr->set_buf[size] = '\0';
-	ret = kstrtoull(attr->set_buf, 0, &val);
-	if (ret)
+	ret = sscanf(attr->set_buf, attr->fmt, &val);
+	if (ret != 1) {
+		ret = -EINVAL;
 		goto out;
+	}
+
 	ret = attr->set(attr->data, val);
 	if (ret == 0)
 		ret = len; /* on success, claim we got the whole input */
-- 
2.37.1

Re: [PATCH] libfs: fix error format in simple_attr_write()

[Andrew Morton]

On Thu, 15 Sep 2022 09:15:44 +0000 Eliav Farber <farbere@amazon.com> wrote:

> In commit 488dac0c9237 ("libfs: fix error cast of negative value in
> simple_attr_write()"), simple_attr_write() was changed to use kstrtoull()
> instead of simple_strtoll() to convert a string got from a user.
> A user trying to set a negative value will get an error.
> 
> This is wrong since it breaks all the places that use
> DEFINE_DEBUGFS_ATTRIBUTE() with format of a signed integer.
> 
> For the record there are 43 current users of signed integer which are
> likely to be effected by this:
> 
> $ git grep -n -A1 -w DEFINE_DEBUGFS_ATTRIBUTE | grep ');' |
> sed 's,.*\(".*%.*"\).*,\1,' | sort | uniq -c
>   1 "%08llx\n"
>   5 "0x%016llx\n"
>   5 "0x%02llx\n"
>   5 "0x%04llx\n"
>  13 "0x%08llx\n"
>   1 "0x%4.4llx\n"
>   3 "0x%.4llx\n"
>   4 "0x%llx\n"
>   1 "%1lld\n"
>  40 "%lld\n"
>   2 "%lli\n"
> 129 "%llu\n"
>   1 "%#llx\n"
>   2 "%llx\n"
> 
> u64 is not an issue for negative numbers.
> The %lld and %llu in any case are for 64-bit value, representing it as
> unsigned simplifies the generic code, but it doesn't mean we can't keep
> their signed value if we know that.
> 
> This change uses sscanf() to fix the problem since it does the conversion
> based on the supplied format string.
> 
> Fixes: 488dac0c9237 ("libfs: fix error cast of negative value in simple_attr_write()")

488dac0c9237 was two years ago, so I'm assuming that this error isn't
causing a lot of trouble out there.

In which I may be totally wrong.  Do you see a reason for backporting
this fix into earlier kernels?  If so, what is it?

Thanks.