On Wed, May 26, 2021 at 05:12:49PM +0100, Will Deacon wrote:

On Wed, May 26, 2021 at 05:15:51PM +0200, Peter Zijlstra wrote:

quoted

On Tue, May 25, 2021 at 04:14:20PM +0100, Will Deacon wrote:

quoted

Reject explicit requests to change the affinity mask of a task via
set_cpus_allowed_ptr() if the requested mask is not a subset of the
mask returned by task_cpu_possible_mask(). This ensures that the
'cpus_mask' for a given task cannot contain CPUs which are incapable of
executing it, except in cases where the affinity is forced.

Reviewed-by: Quentin Perret <redacted>
Signed-off-by: Will Deacon <will@kernel.org>
---
 kernel/sched/core.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 00ed51528c70..8ca7854747f1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c

@@ -2346,6 +2346,7 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
 				  u32 flags)
 {
 	const struct cpumask *cpu_valid_mask = cpu_active_mask;
+	const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p);
 	unsigned int dest_cpu;
 	struct rq_flags rf;
 	struct rq *rq;

@@ -2366,6 +2367,9 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
 		 * set_cpus_allowed_common() and actually reset p->cpus_ptr.
 		 */
 		cpu_valid_mask = cpu_online_mask;
+	} else if (!cpumask_subset(new_mask, cpu_allowed_mask)) {
+		ret = -EINVAL;
+		goto out;
 	}

So what about the case where the 32bit task is in-kernel and in
migrate-disable ? surely we ought to still validate the new mask against
task_cpu_possible_mask.

That's a good question.

Given that 32-bit tasks in the kernel are running in 64-bit mode, we can
actually tolerate them moving around arbitrarily as long as they _never_ try
to return to userspace on a 64-bit-only CPU. I think this should be the case
as long as we don't try to return to userspace with migration disabled, no?

Consider:

	8 CPUs, lower 4 have 32bit, higher 4 do not

	A - a 32 bit task		B

	sys_foo()
	  migrate_disable()
	  				sys_sched_setaffinity(A, 0xf0)
					  if (.. | migration_disabled(A))
					    // not checking nothing

					  __do_set_cpus_allowed();

	  migrate_enable()
	    __set_cpus_allowed(SCA_MIGRATE_ENABLE)
	      // frob outselves somewhere in 0xf0
	  sysret
	  *BOOM*


That is, I'm thinking we ought to disallow that sched_setaffinity() with
-EINVAL for 0xf0 has no intersection with 0x0f.