[PATCH] badblocks: fix infinite loop due to incorrect rounding and overflow

[PATCH] badblocks: fix infinite loop due to incorrect rounding and overflow

[Ramesh Adhikari]

The roundup() and rounddown() macros return the rounded value but
do not modify the input in place. In _badblocks_set(), _badblocks_clear(),
and badblocks_check(), the return values were being discarded, causing
s and target/next to remain unrounded. This resulted in sectors
being calculated from unrounded values, which could lead to sectors
being way too large (or zero), causing infinite loops in the
re_insert/re_clear/re_check loops.

Additionally, add integer overflow checks (s > ULLONG_MAX - sectors)
before the s + sectors calculation in all three functions to prevent
overflow-related issues. Also add early return when sectors becomes
zero after rounding in badblocks_check().

Root cause: When s and sectors have specific values (e.g., from
syzkaller fuzzing via nvdimm ioctl), the unrounded values cause
sectors to be incorrectly calculated. In _badblocks_clear(), this
could result in needing 2^46 iterations to process 2^55 sectors,
triggering RCU stall warnings and effectively hanging the kernel.

Fix by properly capturing the return values from roundup() and
rounddown(), adding overflow checks before sector arithmetic, and
handling the zero-sectors case in badblocks_check().

Signed-off-by: Ramesh Adhikari <adhikari.resume@gmail.com>
---
 block/badblocks.c | 43 ++++++++++++++++++++++++++++++++++---------
 1 file changed, 34 insertions(+), 9 deletions(-)

diff --git a/block/badblocks.c b/block/badblocks.c
index ece64e76fe8..a5ffae65a05 100644
--- a/block/badblocks.c
+++ b/block/badblocks.c
@@ -855,13 +855,21 @@ static bool _badblocks_set(struct badblocks *bb, sector_t s, sector_t sectors,
 
 	if (bb->shift) {
 		/* round the start down, and the end up */
+		if (s > ULLONG_MAX - sectors)
+			return false;
 		sector_t next = s + sectors;
 
-		rounddown(s, 1 << bb->shift);
-		roundup(next, 1 << bb->shift);
-		sectors = next - s;
+		s = rounddown(s, 1 << bb->shift);
+		next = roundup(next, 1 << bb->shift);
+		if (next < s)
+			sectors = 0;
+		else
+			sectors = next - s;
 	}
 
+	if (sectors == 0)
+		return false;
+
 	write_seqlock_irqsave(&bb->lock, flags);
 
 	bad.ack = acknowledged;
@@ -1070,12 +1078,20 @@ static bool _badblocks_clear(struct badblocks *bb, sector_t s, sector_t sectors)
 		 * However it is better the think a block is bad when it
 		 * isn't than to think a block is not bad when it is.
 		 */
+		if (s > ULLONG_MAX - sectors)
+			return false;
 		target = s + sectors;
-		roundup(s, 1 << bb->shift);
-		rounddown(target, 1 << bb->shift);
-		sectors = target - s;
+		s = roundup(s, 1 << bb->shift);
+		target = rounddown(target, 1 << bb->shift);
+		if (target < s)
+			sectors = 0;
+		else
+			sectors = target - s;
 	}
 
+	if (sectors == 0)
+		return false;
+
 	write_seqlock_irq(&bb->lock);
 
 	bad.ack = true;
@@ -1305,11 +1321,20 @@ int badblocks_check(struct badblocks *bb, sector_t s, sector_t sectors,
 
 	if (bb->shift > 0) {
 		/* round the start down, and the end up */
+		if (s > ULLONG_MAX - sectors) {
+			return -EINVAL;
+		}
 		sector_t target = s + sectors;
 
-		rounddown(s, 1 << bb->shift);
-		roundup(target, 1 << bb->shift);
-		sectors = target - s;
+		s = rounddown(s, 1 << bb->shift);
+		target = roundup(target, 1 << bb->shift);
+		if (target < s)
+			sectors = 0;
+		else
+			sectors = target - s;
+
+		if (sectors == 0)
+			return 0;
 	}
 
 retry:
-- 
2.43.0

Re: [PATCH] badblocks: fix infinite loop due to incorrect rounding and overflow

[Greg KH]

On Mon, Apr 27, 2026 at 08:40:48PM +0530, Ramesh Adhikari wrote:
> The roundup() and rounddown() macros return the rounded value but
> do not modify the input in place. In _badblocks_set(), _badblocks_clear(),
> and badblocks_check(), the return values were being discarded, causing
> s and target/next to remain unrounded. This resulted in sectors
> being calculated from unrounded values, which could lead to sectors
> being way too large (or zero), causing infinite loops in the
> re_insert/re_clear/re_check loops.
> 
> Additionally, add integer overflow checks (s > ULLONG_MAX - sectors)
> before the s + sectors calculation in all three functions to prevent
> overflow-related issues. Also add early return when sectors becomes
> zero after rounding in badblocks_check().
> 
> Root cause: When s and sectors have specific values (e.g., from
> syzkaller fuzzing via nvdimm ioctl), the unrounded values cause
> sectors to be incorrectly calculated. In _badblocks_clear(), this
> could result in needing 2^46 iterations to process 2^55 sectors,
> triggering RCU stall warnings and effectively hanging the kernel.
> 
> Fix by properly capturing the return values from roundup() and
> rounddown(), adding overflow checks before sector arithmetic, and
> handling the zero-sectors case in badblocks_check().
> 
> Signed-off-by: Ramesh Adhikari <adhikari.resume@gmail.com>
> ---
>  block/badblocks.c | 43 ++++++++++++++++++++++++++++++++++---------
>  1 file changed, 34 insertions(+), 9 deletions(-)
> 
> diff --git a/block/badblocks.c b/block/badblocks.c
> index ece64e76fe8..a5ffae65a05 100644
> --- a/block/badblocks.c
> +++ b/block/badblocks.c
> @@ -855,13 +855,21 @@ static bool _badblocks_set(struct badblocks *bb, sector_t s, sector_t sectors,
>  
>  	if (bb->shift) {
>  		/* round the start down, and the end up */
> +		if (s > ULLONG_MAX - sectors)
> +			return false;
>  		sector_t next = s + sectors;
>  
> -		rounddown(s, 1 << bb->shift);
> -		roundup(next, 1 << bb->shift);
> -		sectors = next - s;
> +		s = rounddown(s, 1 << bb->shift);
> +		next = roundup(next, 1 << bb->shift);
> +		if (next < s)
> +			sectors = 0;
> +		else
> +			sectors = next - s;
>  	}
>  
> +	if (sectors == 0)
> +		return false;
> +
>  	write_seqlock_irqsave(&bb->lock, flags);
>  
>  	bad.ack = acknowledged;
> @@ -1070,12 +1078,20 @@ static bool _badblocks_clear(struct badblocks *bb, sector_t s, sector_t sectors)
>  		 * However it is better the think a block is bad when it
>  		 * isn't than to think a block is not bad when it is.
>  		 */
> +		if (s > ULLONG_MAX - sectors)
> +			return false;
>  		target = s + sectors;
> -		roundup(s, 1 << bb->shift);
> -		rounddown(target, 1 << bb->shift);
> -		sectors = target - s;
> +		s = roundup(s, 1 << bb->shift);
> +		target = rounddown(target, 1 << bb->shift);
> +		if (target < s)
> +			sectors = 0;
> +		else
> +			sectors = target - s;
>  	}
>  
> +	if (sectors == 0)
> +		return false;
> +
>  	write_seqlock_irq(&bb->lock);
>  
>  	bad.ack = true;
> @@ -1305,11 +1321,20 @@ int badblocks_check(struct badblocks *bb, sector_t s, sector_t sectors,
>  
>  	if (bb->shift > 0) {
>  		/* round the start down, and the end up */
> +		if (s > ULLONG_MAX - sectors) {
> +			return -EINVAL;
> +		}
>  		sector_t target = s + sectors;
>  
> -		rounddown(s, 1 << bb->shift);
> -		roundup(target, 1 << bb->shift);
> -		sectors = target - s;
> +		s = rounddown(s, 1 << bb->shift);
> +		target = roundup(target, 1 << bb->shift);
> +		if (target < s)
> +			sectors = 0;
> +		else
> +			sectors = target - s;
> +
> +		if (sectors == 0)
> +			return 0;
>  	}
>  
>  retry:
> -- 
> 2.43.0
> 

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
a patch that has triggered this response.  He used to manually respond
to these common problems, but in order to save his sanity (he kept
writing the same thing over and over, yet to different people), I was
created.  Hopefully you will not take offence and will fix the problem
in your patch and resubmit it so that it can be accepted into the Linux
kernel tree.

You are receiving this message because of the following common error(s)
as indicated below:

- This looks like a new version of a previously submitted patch, but you
  did not list below the --- line any changes from the previous version.
  Please read the section entitled "The canonical patch format" in the
  kernel file, Documentation/process/submitting-patches.rst for what
  needs to be done here to properly describe this.

If you wish to discuss this problem further, or you have questions about
how to resolve this issue, please feel free to respond to this email and
Greg will reply once he has dug out from the pending patches received
from other developers.

thanks,

greg k-h's patch email bot

Re: [PATCH] badblocks: fix infinite loop due to incorrect rounding and overflow

[kernel test robot]

Hi Ramesh,

kernel test robot noticed the following build errors:

[auto build test ERROR on axboe/for-next]
[also build test ERROR on linus/master v7.1-rc1 next-20260429]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Ramesh-Adhikari/badblocks-fix-infinite-loop-due-to-incorrect-rounding-and-overflow/20260429-044425
base:   https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux.git for-next
patch link:    https://lore.kernel.org/r/20260427151048.756072-1-adhikari.resume%40gmail.com
patch subject: [PATCH] badblocks: fix infinite loop due to incorrect rounding and overflow
config: arm-randconfig-002-20260430 (https://download.01.org/0day-ci/archive/20260430/202604300729.gKrAMFdE-lkp@intel.com/config)
compiler: arm-linux-gnueabi-gcc (GCC) 8.5.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260430/202604300729.gKrAMFdE-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202604300729.gKrAMFdE-lkp@intel.com/

All errors (new ones prefixed by >>):

   arm-linux-gnueabi-ld: block/badblocks.o: in function `badblocks_clear':
>> badblocks.c:(.text+0x1d18): undefined reference to `__aeabi_uldivmod'
>> arm-linux-gnueabi-ld: badblocks.c:(.text+0x1d54): undefined reference to `__aeabi_uldivmod'
   arm-linux-gnueabi-ld: block/badblocks.o: in function `_badblocks_set':
   badblocks.c:(.text+0x2c90): undefined reference to `__aeabi_uldivmod'
   arm-linux-gnueabi-ld: badblocks.c:(.text+0x2ce0): undefined reference to `__aeabi_uldivmod'
   arm-linux-gnueabi-ld: block/badblocks.o: in function `badblocks_check':
   badblocks.c:(.text+0x3fb8): undefined reference to `__aeabi_uldivmod'
   arm-linux-gnueabi-ld: block/badblocks.o:badblocks.c:(.text+0x4018): more undefined references to `__aeabi_uldivmod' follow

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] badblocks: fix infinite loop due to incorrect rounding and overflow

[kernel test robot]

Hi Ramesh,

kernel test robot noticed the following build errors:

[auto build test ERROR on axboe/for-next]
[also build test ERROR on linus/master v7.1-rc1 next-20260429]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Ramesh-Adhikari/badblocks-fix-infinite-loop-due-to-incorrect-rounding-and-overflow/20260429-044425
base:   https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux.git for-next
patch link:    https://lore.kernel.org/r/20260427151048.756072-1-adhikari.resume%40gmail.com
patch subject: [PATCH] badblocks: fix infinite loop due to incorrect rounding and overflow
config: i386-allnoconfig (https://download.01.org/0day-ci/archive/20260430/202604301231.IpPh4AiH-lkp@intel.com/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260430/202604301231.IpPh4AiH-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202604301231.IpPh4AiH-lkp@intel.com/

All errors (new ones prefixed by >>):

   ld: block/badblocks.o: in function `badblocks_check':
   badblocks.c:(.text+0x10af): undefined reference to `__umoddi3'
>> ld: badblocks.c:(.text+0x1109): undefined reference to `__umoddi3'
   ld: block/badblocks.o: in function `_badblocks_set':
   badblocks.c:(.text+0x12b0): undefined reference to `__umoddi3'
   ld: badblocks.c:(.text+0x12f4): undefined reference to `__umoddi3'
   ld: block/badblocks.o: in function `_badblocks_clear':
   badblocks.c:(.text+0x1bfc): undefined reference to `__umoddi3'
   ld: block/badblocks.o:badblocks.c:(.text+0x1c1c): more undefined references to `__umoddi3' follow

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki