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Sean Christopherson 723acd75a0 perf: Protect perf_guest_cbs with RCU commit `ff083a2d97` upstream. Protect perf_guest_cbs with RCU to fix multiple possible errors. Luckily, all paths that read perf_guest_cbs already require RCU protection, e.g. to protect the callback chains, so only the direct perf_guest_cbs touchpoints need to be modified. Bug #1 is a simple lack of WRITE_ONCE/READ_ONCE behavior to ensure perf_guest_cbs isn't reloaded between a !NULL check and a dereference. Fixed via the READ_ONCE() in rcu_dereference(). Bug #2 is that on weakly-ordered architectures, updates to the callbacks themselves are not guaranteed to be visible before the pointer is made visible to readers. Fixed by the smp_store_release() in rcu_assign_pointer() when the new pointer is non-NULL. Bug #3 is that, because the callbacks are global, it's possible for readers to run in parallel with an unregisters, and thus a module implementing the callbacks can be unloaded while readers are in flight, resulting in a use-after-free. Fixed by a synchronize_rcu() call when unregistering callbacks. Bug #1 escaped notice because it's extremely unlikely a compiler will reload perf_guest_cbs in this sequence. perf_guest_cbs does get reloaded for future derefs, e.g. for ->is_user_mode(), but the ->is_in_guest() guard all but guarantees the consumer will win the race, e.g. to nullify perf_guest_cbs, KVM has to completely exit the guest and teardown down all VMs before KVM start its module unload / unregister sequence. This also makes it all but impossible to encounter bug #3. Bug #2 has not been a problem because all architectures that register callbacks are strongly ordered and/or have a static set of callbacks. But with help, unloading kvm_intel can trigger bug #1 e.g. wrapping perf_guest_cbs with READ_ONCE in perf_misc_flags() while spamming kvm_intel module load/unload leads to: BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 0 P4D 0 Oops: 0000 [#1] PREEMPT SMP CPU: 6 PID: 1825 Comm: stress Not tainted 5.14.0-rc2+ #459 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:perf_misc_flags+0x1c/0x70 Call Trace: perf_prepare_sample+0x53/0x6b0 perf_event_output_forward+0x67/0x160 __perf_event_overflow+0x52/0xf0 handle_pmi_common+0x207/0x300 intel_pmu_handle_irq+0xcf/0x410 perf_event_nmi_handler+0x28/0x50 nmi_handle+0xc7/0x260 default_do_nmi+0x6b/0x170 exc_nmi+0x103/0x130 asm_exc_nmi+0x76/0xbf Fixes: `39447b386c` ("perf: Enhance perf to allow for guest statistic collection from host") Signed-off-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20211111020738.2512932-2-seanjc@google.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>		2022-01-20 09:17:50 +01:00
arch	perf: Protect perf_guest_cbs with RCU	2022-01-20 09:17:50 +01:00
block	iocost: Fix divide-by-zero on donation from low hweight cgroup	2021-12-22 09:30:57 +01:00
certs	certs: Trigger creation of RSA module signing key if it's not an RSA key	2021-09-15 09:50:29 +02:00
crypto	crypto: pcrypt - Delay write to padata->info	2021-11-18 14:04:12 +01:00
Documentation	bpf: Add kconfig knob for disabling unpriv bpf by default	2022-01-05 12:40:34 +01:00
drivers	remoteproc: qcom: pil_info: Don't memcpy_toio more than is provided	2022-01-20 09:17:50 +01:00
fs	vfs: fs_context: fix up param length parsing in legacy_parse_param	2022-01-20 09:17:50 +01:00
include	perf: Protect perf_guest_cbs with RCU	2022-01-20 09:17:50 +01:00
init	bpf: Add kconfig knob for disabling unpriv bpf by default	2022-01-05 12:40:34 +01:00
ipc	shm: extend forced shm destroy to support objects from several IPC nses	2021-12-01 09:19:10 +01:00
kernel	perf: Protect perf_guest_cbs with RCU	2022-01-20 09:17:50 +01:00
lib	siphash: use _unaligned version by default	2021-12-08 09:03:23 +01:00
LICENSES	LICENSES/deprecated: add Zlib license text	2020-09-16 14:33:49 +02:00
mm	mm: mempolicy: fix THP allocations escaping mempolicy restrictions	2021-12-29 12:26:06 +01:00
net	can: isotp: convert struct tpcon::{idx,len} to unsigned int	2022-01-16 09:14:23 +01:00
samples	ftrace/samples: Add missing prototypes direct functions	2022-01-11 15:25:00 +01:00
scripts	recordmcount.pl: fix typo in s390 mcount regex	2022-01-05 12:40:29 +01:00
security	selinux: initialize proto variable in selinux_ip_postroute_compat()	2022-01-05 12:40:30 +01:00
sound	ASoC: rt5682: fix the wrong jack type detected	2021-12-29 12:26:07 +01:00
tools	selftests: x86: fix [-Wstringop-overread] warn in test_process_vm_readv()	2022-01-11 15:24:58 +01:00
usr	Merge branch 'work.fdpic' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs	2020-08-07 13:29:39 -07:00
virt	KVM: downgrade two BUG_ONs to WARN_ON_ONCE	2021-12-22 09:30:50 +01:00
.clang-format	RDMA 5.10 pull request	2020-10-17 11:18:18 -07:00
.cocciconfig
.get_maintainer.ignore	Opt out of scripts/get_maintainer.pl	2019-05-16 10:53:40 -07:00
.gitattributes	.gitattributes: use 'dts' diff driver for dts files	2019-12-04 19:44:11 -08:00
.gitignore	kbuild: generate Module.symvers only when vmlinux exists	2021-05-19 10:12:59 +02:00
.mailmap	mailmap: add two more addresses of Uwe Kleine-König	2020-12-06 10:19:07 -08:00
COPYING	COPYING: state that all contributions really are covered by this file	2020-02-10 13:32:20 -08:00
CREDITS	MAINTAINERS: Move Jason Cooper to CREDITS	2020-11-30 10:20:34 +01:00
Kbuild	kbuild: rename hostprogs-y/always to hostprogs/always-y	2020-02-04 01:53:07 +09:00
Kconfig	kbuild: ensure full rebuild when the compiler is updated	2020-05-12 13:28:33 +09:00
MAINTAINERS	MAINTAINERS: adjust GCC PLUGINS after gcc-plugin.sh removal	2021-12-14 11:32:46 +01:00
Makefile	kbuild: Add $(KBUILD_HOSTLDFLAGS) to 'has_libelf' test	2022-01-20 09:17:49 +01:00
README	Drop all 00-INDEX files from Documentation/	2018-09-09 15:08:58 -06:00

README

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.