[PATCH] sched/isolation: don't do unbounded chomp on bootarg string

[Paul Gortmaker]

After commit 3662daf023500dc084fa3b96f68a6f46179ddc73
("sched/isolation: Allow "isolcpus=" to skip unknown sub-parameters")
the isolcpus= string is walked to skip over what might be future flag
comma separated additions.

However, there is a logic error, and so as can clearly be seen below, it
will ignore its own arg len and search to the end of the bootarg string.

 $ dmesg|grep isol
 Command line: BOOT_IMAGE=/boot/bzImage isolcpus=xyz pleasedontparseme=1 root=/dev/sda1 ro
 Kernel command line: BOOT_IMAGE=/boot/bzImage isolcpus=xyz pleasedontparseme=1 root=/dev/sda1 ro
 isolcpus: Skipped unknown flag xyz
 isolcpus: Invalid flag pleasedontparseme=1 root=/dev/sda1 ro

This happens because the flag "skip" code does an unconditional
increment, which skips over the '\0' check the loop body looks for. If
the isolcpus= happens to be the last bootarg, then you'd never notice?

So we only increment if the skipped flag is followed by a comma, as per
what the existing "continue" flag matching code does.

Note that isolcpus= was declared deprecated as of v4.15 (b0d40d2b22fe),
so we might want to revisit that if we are trying to future-proof it
as recently as a year ago for as yet unseen new flags.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Fixes: 3662daf02350 ("sched/isolation: Allow "isolcpus=" to skip unknown sub-parameters")
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 5a6ea03f9882..9652dba7e938 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -188,7 +188,8 @@ static int __init housekeeping_isolcpus_setup(char *str)
 		}
 
 		pr_info("isolcpus: Skipped unknown flag %.*s\n", len, par);
-		str++;
+		if (str[1] == ',')	/* above continue; match on "flag," */
+			str++;
 	}
 
 	/* Default behaviour for isolcpus without flags */
-- 
2.25.1