Commit cbd7e6ca authored by Christophe Leroy's avatar Christophe Leroy Committed by Michael Ellerman
Browse files

powerpc/fault: Avoid heavy search_exception_tables() verification 



search_exception_tables() is an heavy operation, we have to avoid it.
When KUAP is selected, we'll know the fault has been blocked by KUAP.
When it is blocked by KUAP, check whether we are in an expected
userspace access place. If so, emit a warning to spot something is
going work. Otherwise, just remain silent, it will likely Oops soon.

When KUAP is not selected, it behaves just as if the address was
already in the TLBs and no fault was generated.

Signed-off-by: default avatarChristophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: default avatarNicholas Piggin <npiggin@gmail.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9870f01e293a5a76c4f4e4ddd4a6b0f63038c591.1607491748.git.christophe.leroy@csgroup.eu
parent 3dc12dfe

arch/powerpc/mm/fault.c

+13 −15
Original line number Diff line number Diff line
@@ -210,28 +210,26 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code, 
		return true;

	}



	if (!is_exec && address < TASK_SIZE && (error_code & (DSISR_PROTFAULT | DSISR_KEYFAULT)) &&

	    !search_exception_tables(regs->nip)) {

		pr_crit_ratelimited("Kernel attempted to access user page (%lx) - exploit attempt? (uid: %d)\n",

				    address,

				    from_kuid(&init_user_ns, current_uid()));

	}



	// Kernel fault on kernel address is bad

	if (address >= TASK_SIZE)

		return true;



	// Read/write fault blocked by KUAP is bad, it can never succeed.

	if (bad_kuap_fault(regs, address, is_write)) {

		pr_crit_ratelimited("Kernel attempted to %s user page (%lx) - exploit attempt? (uid: %d)\n",

				    is_write ? "write" : "read", address,

				    from_kuid(&init_user_ns, current_uid()));



		// Fault on user outside of certain regions (eg. copy_tofrom_user()) is bad

		if (!search_exception_tables(regs->nip))

			return true;



		// Read/write fault in a valid region (the exception table search passed

		// above), but blocked by KUAP is bad, it can never succeed.

	if (bad_kuap_fault(regs, address, is_write))

		return WARN(true, "Bug: %s fault blocked by KUAP!", is_write ? "Write" : "Read");

	}



	// What's left? Kernel fault on user in well defined regions (extable

	// matched), and allowed by KUAP in the faulting context.

	// What's left? Kernel fault on user and allowed by KUAP in the faulting context.

	return false;

}

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