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powerpc/fault: Avoid heavy search_exception_tables() verification

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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 <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9870f01e293a5a76c4f4e4ddd4a6b0f63038c591.1607491748.git.christophe.leroy@csgroup.eu
This commit is contained in:
Christophe Leroy 2020-12-09 05:29:24 +00:00 committed by Michael Ellerman
parent 3dc12dfe74
commit cbd7e6ca02
1 changed files with 13 additions and 15 deletions
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28
arch/powerpc/mm/fault.c
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@ -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;
}