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 <[email protected]>
Signed-off-by: Gregory Price <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Acked-by: Oleg Nesterov <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Link: https://lore.kernel.org/r/[email protected]

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