Re: [PATCH v3] sysctl: fix check against uninitialized variable in proc_do_large_bitmap

[Kees Cook]

On Thu, Mar 19, 2026 at 11:50:50PM +0100, Marc Buerg wrote:
> proc_do_large_bitmap() does not initialize variable c, which is expected
> to be set to a trailing character by proc_get_long().
> 
> However, proc_get_long() only sets c when the input buffer contains a
> trailing character after the parsed value.
> 
> If c is not initialized it may happen to contain a '-'. If this is the
> case proc_do_large_bitmap() expects to be able to parse a second part of
> the input buffer. If there is no second part an unjustified -EINVAL will
> be returned.
> 
> Add check that left is non-zero before checking c, as proc_get_long()
> ensures that the passed left is non-zero, if a trailing character
> exists.
> 
> ---

Please don't include "---" as a separator here: we want to keep your
entire commit log (including the SoB and other tags).

Also, I think the explicit zero-init of 'c' would be nice to keep just
for robustness: the compiler can elide it if it decides it's a duplicate
store. There's only upside to including it.

-Kees

> When writing to /proc/sys/net/ipv4/ip_local_reserved_ports it is
> possible to receive an -EINVAL for a valid value.
> 
> This happens due to a check of a potentially uninitialized variable in
> the proc_do_large_bitmap() function, namely char c. To trigger this
> behavior the variable has to contain the later explicitly checked '-'
> char by chance.
> 
> In proc_do_large_bitmap() it is expected that the variable might be
> filled by the proc_get_long() function with the trailing character of
> the given input. But only if a trailing character exists within the
> passed size of the buffer.
> 
> If no trailing character is present we still do a c == '-' check. If the
> uninitialized variable contains this char the function continues
> parsing. It will now set err to -EINVAL in the next proc_get_long()
> call, as there is nothing more to parse.
> 
> proc_do_large_bitmap() passes left to the proc_get_long() call. left
> will only be non-zero, if a trailing character has been written.
> Therefore, checking that left is non-zero before accessing c fixes this
> problem.
> 
> The problem will only arise sporadically, as the variable must contain
> '-' by chance. On the affected system CONFIG_INIT_STACK_NONE=y was
> enabled. Further, when enabling eBPF tracing to dump contents of the
> stack the issue disappeared.
> 
> Fixes: 9f977fb7ae9d ("sysctl: add proc_do_large_bitmap")
> Signed-off-by: Marc Buerg <buermarc@xxxxxxxxxxxxxx>
> Reviewed-by: Peter Seiderer <ps.report@xxxxxxx>
> ---
> Changes in v3:
> - Add Reviewed-by: Peter Seiderer <ps.report@xxxxxxx>
> - Re-include bug context into cover letter
> - Link to v2: https://lore.kernel.org/r/20260317-fix-uninitialized-variable-in-proc_do_large_bitmap-v2-1-6dfb1aefa287@xxxxxxxxxxxxxx
> 
> Changes in v2:
> - Drop initialization of c to 0
> - Include checking that left is non-zero before checking against c
> - Link to v1: https://lore.kernel.org/r/20260312-fix-uninitialized-variable-in-proc_do_large_bitmap-v1-1-35ad2dddaf21@xxxxxxxxxxxxxx
> ---
>  kernel/sysctl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 9d3a666ffde1..dd337a63da41 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1171,7 +1171,7 @@ int proc_do_large_bitmap(const struct ctl_table *table, int dir,
>  				left--;
>  			}
>  
> -			if (c == '-') {
> +			if (left && c == '-') {
>  				err = proc_get_long(&p, &left, &val_b,
>  						     &neg, tr_b, sizeof(tr_b),
>  						     &c);
> 
> ---
> base-commit: 80234b5ab240f52fa45d201e899e207b9265ef91
> change-id: 20260312-fix-uninitialized-variable-in-proc_do_large_bitmap-30c6ef4ac1c5
> 
> Best regards,
> -- 
> buermarc <buermarc@xxxxxxxxxxxxxx>
> 

-- 
Kees Cook