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Mon, 09 Jul 2018 22:42:01 +0000 Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by aserv0021.oracle.com (8.14.4/8.14.4) with ESMTP id w69Mg01s014479 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 9 Jul 2018 22:42:00 GMT Received: from abhmp0001.oracle.com (abhmp0001.oracle.com [141.146.116.7]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id w69Mg0N1017190; Mon, 9 Jul 2018 22:42:00 GMT Received: from dhcp-burlington7-2nd-B-east-10-152-55-162.usdhcp.oraclecorp.com (/10.152.32.65) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Mon, 09 Jul 2018 15:42:00 -0700 Subject: Re: [RFC PATCH] perf/core: don't sample kernel regs upon skid To: Mark Rutland , Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Jin Yao References: <20180702151250.14536-1-mark.rutland@arm.com> <20180702154655.GR2494@hirez.programming.kicks-ass.net> <20180702160249.qck45h76galxrckn@lakrids.cambridge.arm.com> From: Boris Ostrovsky Openpgp: preference=signencrypt Autocrypt: addr=boris.ostrovsky@oracle.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <20180702160249.qck45h76galxrckn@lakrids.cambridge.arm.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=8949 signatures=668705 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1807090255 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/02/2018 12:02 PM, Mark Rutland wrote: > On Mon, Jul 02, 2018 at 05:46:55PM +0200, Peter Zijlstra wrote: >> On Mon, Jul 02, 2018 at 04:12:50PM +0100, Mark Rutland wrote: >>> Users can request that general purpose registers, instruction pointer, >>> etc, are sampled when a perf event counter overflows. To try to avoid >>> this resulting in kernel state being leaked, unprivileged users are >>> usually forbidden from opening events which count while the kernel is >>> running. >>> >>> Unfortunately, this is not sufficient to avoid leading kernel state. >> 'leaking' surely. >> >>> For various reasons, there can be a delay between the overflow occurring >>> and the resulting overflow exception (e.g. an NMI) being taken. During >>> this window, other instructions may be executed, resulting in skid. >>> >>> This skid means that a userspace-only event overflowing may result in an >>> exception being taken *after* entry to the kernel, allowing kernel >>> registers to be sampled. Depending on the amount of skid, this may only >>> leak the PC (breaking KASLR), or it may leak secrets which are currently >>> live in GPRs. >>> >>> Let's avoid this by only sampling from the user registers when an event >>> is supposed to exclude the kernel, providing the illusion that the >>> overflow exception is taken from userspace. >>> >>> We also have similar cases when sampling a guest, where we get the host >>> regs in some cases. It's not entirely clear to me how we should handle >>> these. >> Would not a similar: >> >> if ((event->attr.exclude_hv || event->attr.exclude_host) /* WTF both !? */ && >> perf_guest_cbs && !perf_guest_cbs->is_in_guest()) >> return perf_guest_cbs->guest_pt_regs(); >> >> work there? > Mostly. It's fun if the user also passed exclude_guest -- I have no idea > what should be sampled there, if anything. > > It's easy to get stuck in a rabbit hole looking at this. > >> Of course, perf_guest_info_callbacks is currently lacking that >> guest_pt_regs() thingy.. > Yeah; I started looking at implementing it, but ran away since it wasn't > clear to me how to build that on most architectures. > >>> Signed-off-by: Mark Rutland >>> Cc: Ingo Molnar >>> Cc: Jin Yao >>> Cc: Peter Zijlstra >>> --- >>> kernel/events/core.c | 28 ++++++++++++++++++++++++++++ >>> 1 file changed, 28 insertions(+) >>> >>> diff --git a/kernel/events/core.c b/kernel/events/core.c >>> index 8f0434a9951a..2ab2548b2e66 100644 >>> --- a/kernel/events/core.c >>> +++ b/kernel/events/core.c >>> @@ -6361,6 +6361,32 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs) >>> return callchain ?: &__empty_callchain; >>> } >>> >>> +static struct pt_regs *perf_get_sample_regs(struct perf_event *event, >>> + struct pt_regs *regs) >>> +{ >>> + /* >>> + * Due to interrupt latency (AKA "skid"), we may enter the kernel >>> + * before taking an overflow, even if the PMU is only counting user >>> + * events. >>> + * >>> + * If we're not counting kernel events, always use the user regs when >>> + * sampling. >>> + * >>> + * TODO: what do we do about sampling a guest's registers? The IP is >>> + * special-cased, but for the rest of the regs they'll get the >>> + * user/kernel regs depending on whether exclude_kernel is set, which >>> + * is nonsensical. >>> + * >>> + * We can't get at the full set of regs in all cases (e.g. Xen's PV PMU >>> + * can't provide the GPRs), so should we just zero the GPRs when in a >>> + * guest? Or skip outputting the regs in perf_output_sample? >> Seems daft Xen cannot provide registers; why is that? Boris? > The xen_pmu_regs structure simply doesn't have them, so I assume there's > no API to get them. > > Given we don't currently sample the guest regs, I'd be tempted to just > zero them for now, or skip the sample at output time (if that doesn't > break some other case). (Was out on vacation, couldn't respond earlier) Yes, PV guests only get a limited set of registers passed to the handler by the hypervisor. GPRs are not part of this set. I think we need do diff --git a/arch/x86/xen/pmu.c b/arch/x86/xen/pmu.c index 7d00d4a..95997e6 100644 --- a/arch/x86/xen/pmu.c +++ b/arch/x86/xen/pmu.c @@ -478,7 +478,7 @@ static void xen_convert_regs(const struct xen_pmu_regs *xen_regs,  irqreturn_t xen_pmu_irq_handler(int irq, void *dev_id)  {         int err, ret = IRQ_NONE; -       struct pt_regs regs; +       struct pt_regs regs = {0};         const struct xen_pmu_data *xenpmu_data = get_xenpmu_data();         uint8_t xenpmu_flags = get_xenpmu_flags(); Do you want me to submit a separate patch or can you make this part of yours? Thanks. -boris