syscall_user_dispatch: Untag selector address before access_ok()

To support checkpoint/restart, ptrace must be able to set the selector
of the tracee.  The selector is a user pointer that may be subject to
memory tagging extensions on some architectures (namely ARM MTE).

access_ok() clears memory tags for tagged addresses if the current task has
memory tagging enabled.

This obviously fails when ptrace modifies the selector of a tracee when
tracer and tracee do not have the same memory tagging enabled state.

Solve this by untagging the selector address before handing it to
access_ok(), like other ptrace functions which modify tracee pointers do.

Obviously a tracer can set an invalid selector address for the tracee, but
that's independent of tagging and a general capability of the tracer.

Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Gregory Price <gregory.price@memverge.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/all/ZCWXE04nLZ4pXEtM@arm.com/
Link: https://lore.kernel.org/r/20230407171834.3558-3-gregory.price@memverge.com

diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c
index 22396b2..7f2add4 100644 (file)
--- a/kernel/entry/syscall_user_dispatch.c
+++ b/kernel/entry/syscall_user_dispatch.c
@@ -87,7 +87,16 @@ static int task_set_syscall_user_dispatch(struct task_struct *task, unsigned lon
                if (offset && offset + len <= offset)
                        return -EINVAL;
 
-               if (selector && !access_ok(selector, sizeof(*selector)))
+               /*
+                * access_ok() will clear memory tags for tagged addresses
+                * if current has memory tagging enabled.
+
+                * To enable a tracer to set a tracees selector the
+                * selector address must be untagged for access_ok(),
+                * otherwise an untagged tracer will always fail to set a
+                * tagged tracees selector.
+                */
+               if (selector && !access_ok(untagged_addr(selector), sizeof(*selector)))
                        return -EFAULT;
 
                break;