Re: [PATCH v4] sysctl: fix uninitialized variable in proc_do_large_bitmap

[Joel Granados <joel.granados@kernel.org>]

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On Wed, Mar 25, 2026 at 11:29:50PM +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.
> 
> Initialize c to 0 to prevent returning -EINVAL on valid input.
> 
> Fixes: 9f977fb7ae9d ("sysctl: add proc_do_large_bitmap")
> Signed-off-by: Marc Buerg <buermarc@googlemail.com>
> ---
> 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 an 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.
> 
> The proc_get_long() function can set c if the length of the parsed long
> is smaller than the given size of the buffer containing the user input.
> This is not the case if the buffer only contains the port value (e.g.
> "123") and sets the size exactly to that (3). Meaning if there is no
> trailing character, c will not be set.
> 
> 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.
> 
> Initializing c to 0 will solve the 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 disappears, which would fit the current explanation as a
> root cause for the observed behavior.
> ---
> Changes in v4:
> - Re-include set c to 0
> - Drop check against left
> - Move trailers
> - Removed Reviewed-by: Peter Seiderer <ps.report@gmx.net>
> - Link to v3: https://lore.kernel.org/r/20260319-fix-uninitialized-variable-in-proc_do_large_bitmap-v3-1-9cfc3ff60c09@googlemail.com
> 
> Changes in v3:
> - Add Reviewed-by: Peter Seiderer <ps.report@gmx.net>
> - 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@googlemail.com
> 
> 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@googlemail.com
> ---
>  kernel/sysctl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 9d3a666ffde1..c9efb17cc255 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1118,7 +1118,7 @@ int proc_do_large_bitmap(const struct ctl_table *table, int dir,
>  	unsigned long bitmap_len = table->maxlen;
>  	unsigned long *bitmap = *(unsigned long **) table->data;
>  	unsigned long *tmp_bitmap = NULL;
> -	char tr_a[] = { '-', ',', '\n' }, tr_b[] = { ',', '\n', 0 }, c;
> +	char tr_a[] = { '-', ',', '\n' }, tr_b[] = { ',', '\n', 0 }, c = 0;
>  
>  	if (!bitmap || !bitmap_len || !left || (*ppos && SYSCTL_KERN_TO_USER(dir))) {
>  		*lenp = 0;
> 
> ---
> base-commit: 80234b5ab240f52fa45d201e899e207b9265ef91
> change-id: 20260312-fix-uninitialized-variable-in-proc_do_large_bitmap-30c6ef4ac1c5
> 
> Best regards,
> -- 
> Marc Buerg <buermarc@googlemail.com>
> 

Thx for the fix

Reviewed-by: Joel Granados <joel.granados@kernel.org>

-- 

Joel Granados

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