powerpc/perf: callchain validate kernel stack pointer bounds

The interrupt frame detection and loads from the hypothetical pt_regs
are not bounds-checked. The next-frame validation only bounds-checks
STACK_FRAME_OVERHEAD, which does not include the pt_regs. Add another
test for this.

The user could set r1 to be equal to the address matching the first
interrupt frame - STACK_INT_FRAME_SIZE, which is in the previous page
due to the kernel redzone, and induce the kernel to load the marker from
there. Possibly this could cause a crash at least. If the user could
induce the previous page to contain a valid marker, then it might be
able to direct perf to read specific memory addresses in a way that
could be transmitted back to the user in the perf data.

Fixes: 20002ded ("perf_counter: powerpc: Add callchain support")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221127124942.1665522-4-npiggin@gmail.com

arch/powerpc/perf/callchain.c

@@ -61,6 +61,7 @@ perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *re
 		next_sp = fp[0];
 
 		if (next_sp == sp + STACK_INT_FRAME_SIZE &&
+		    validate_sp(sp, current, STACK_INT_FRAME_SIZE) &&
 		    fp[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER) {
 			/*
 			 * This looks like an interrupt frame for an