From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
npiggin@gmail.com
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v3 4/5] powerpc/fault: Avoid heavy search_exception_tables() verification
Date: Tue, 08 Dec 2020 20:22:35 +0530 [thread overview]
Message-ID: <87lfe8qrik.fsf@linux.ibm.com> (raw)
In-Reply-To: <731bdee26a5a5c81cd815ed624a6fb3bdef8b4db.1607416578.git.christophe.leroy@csgroup.eu>
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?
>
> - // 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
next prev parent reply other threads:[~2020-12-08 14:54 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-08 8:36 [PATCH v3 1/5] powerpc/book3s64/kuap: Improve error reporting with KUAP Christophe Leroy
2020-12-08 8:36 ` [PATCH v3 2/5] powerpc/mm: sanity_check_fault() should work for all, not only BOOK3S Christophe Leroy
2020-12-08 8:37 ` [PATCH v3 3/5] powerpc/fault: Unnest definition of page_fault_is_write() and page_fault_is_bad() Christophe Leroy
2020-12-08 8:37 ` [PATCH v3 4/5] powerpc/fault: Avoid heavy search_exception_tables() verification Christophe Leroy
2020-12-08 13:00 ` Aneesh Kumar K.V
2020-12-08 14:26 ` Christophe Leroy
2020-12-08 14:31 ` Aneesh Kumar K.V
2020-12-08 14:52 ` Aneesh Kumar K.V [this message]
2020-12-08 15:07 ` Christophe Leroy
2020-12-09 5:34 ` Christophe Leroy
2020-12-08 8:37 ` [PATCH v3 5/5] powerpc/fault: Perform exception fixup in do_page_fault() Christophe Leroy
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