Messages in this thread | | | Subject | Re: [PATCH v3 4/5] powerpc/fault: Avoid heavy search_exception_tables() verification | From | Christophe Leroy <> | Date | Wed, 9 Dec 2020 06:34:21 +0100 |
| |
Le 08/12/2020 à 16:07, Christophe Leroy a écrit : > > > 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 sent out v4 which does that: only emit the warning once we know it is a KUAP fault within an uaccess routine. With that, we should be back more or less as before: warning only if we hit KUAP fault AND it is a place where a userspace access should be granted. We are not anymore in the fast hot path, so calling search_exception_tables() there should be a performance issue.
Christophe
> > I was wondering also if we should keep the WARN() only when CONFIG_PPC_KUAP_DEBUG is set ? >
| |