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

[Marc Buerg <buermarc@googlemail.com>]

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.

---
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@googlemail.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
---
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..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@googlemail.com>