Messages in this thread | | | Subject | Re: [PATCH v3 4/5] powerpc/fault: Avoid heavy search_exception_tables() verification | From | Christophe Leroy <> | Date | Tue, 8 Dec 2020 16:07:32 +0100 |
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Le 08/12/2020 à 15:52, Aneesh Kumar K.V a écrit : > Christophe Leroy <christophe.leroy@csgroup.eu> writes: > >> 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. >> Otherwise, 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> >> --- >> v3: rebased >> v2: Squashed with the preceeding patch which was re-ordering tests that get removed in this patch. >> --- >> arch/powerpc/mm/fault.c | 23 +++++++---------------- >> 1 file changed, 7 insertions(+), 16 deletions(-) >> >> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c >> index 3fcd34c28e10..1770b41e4730 100644 >> --- a/arch/powerpc/mm/fault.c >> +++ b/arch/powerpc/mm/fault.c >> @@ -210,28 +210,19 @@ 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 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)) >> + // 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())); >> return true; >> + } > > > With this I am wondering whether the WARN() in bad_kuap_fault() is > needed. A direct access of userspace address will trigger this, whereas > previously we used bad_kuap_fault() only to identify incorrect restore > of AMR register (ie, to identify kernel bugs). Hence a WARN() there was > useful. We loose that differentiation now?
Yes, I wanted to remove the WARN(), see https://patchwork.ozlabs.org/project/linuxppc-dev/patch/cc9129bdda1dbc2f0a09cf45fece7d0b0e690784.1605541983.git.christophe.leroy@csgroup.eu/ but I understood from Michael that maybe it was not a good idea, so I left it aside for now when rebasing to v3.
Yes previously we were able to differentiate between a direct access of userspace and a valid access triggering a KUAP fault, but at the cost of the heavy search_exception_tables(). The issue was reported by Nick through https://github.com/linuxppc/issues/issues/317
Should be perform the search_exception_tables() once we have hit the KUAP fault and WARN() only in that case ?
I was wondering also if we should keep the WARN() only when CONFIG_PPC_KUAP_DEBUG is set ?
> > >> >> - // 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; >> } >> >> -- >> 2.25.0
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