From cbd7e6ca0210db05c315a27bb5db5a482f2772ce Mon Sep 17 00:00:00 2001 From: Christophe Leroy Date: Wed, 9 Dec 2020 05:29:24 +0000 Subject: [PATCH] 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: Christophe Leroy Reviewed-by: Nicholas Piggin Signed-off-by: Michael Ellerman Link: https://lore.kernel.org/r/9870f01e293a5a76c4f4e4ddd4a6b0f63038c591.1607491748.git.christophe.leroy@csgroup.eu --- arch/powerpc/mm/fault.c | 28 +++++++++++++--------------- 1 file changed, 13 insertions(+), 15 deletions(-) diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c index 04505f938bbc..389a2a875262 100644 --- a/arch/powerpc/mm/fault.c +++ b/arch/powerpc/mm/fault.c @@ -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; - // Fault on user outside of certain regions (eg. copy_tofrom_user()) is bad - if (!search_exception_tables(regs->nip)) - 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())); - // 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)) + // 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. 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; }