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Peter Anvin" , linux-kernel@vger.kernel.org Subject: [PATCH] x86/debug: Forbid tracing arch_uninstall_hw_breakpoint() Date: Mon, 4 Mar 2024 18:43:36 +0100 Message-ID: <20240304174336.68929-1-jannh@google.com> X-Mailer: git-send-email 2.44.0.278.ge034bb2e1d-goog Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1792618516750208349 X-GMAIL-MSGID: 1792618516750208349 I tried using perf_event_open() to set hardware breakpoints on every allowed address between the kernel's _text and _etext. (For reasons.) arch_uninstall_hw_breakpoint() was the only function where this immediately blew up, with DB0 set to the address of the instruction for `dr7 = this_cpu_read(cpu_dr7)`, where the directly preceding instruction does `*slot = NULL;`. I think the issue there is that we clear the breakpoint tracking state before actually disarming the breakpoint. Signed-off-by: Jann Horn --- arch/x86/kernel/hw_breakpoint.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c index b01644c949b2..f46a460dbd31 100644 --- a/arch/x86/kernel/hw_breakpoint.c +++ b/arch/x86/kernel/hw_breakpoint.c @@ -140,8 +140,11 @@ int arch_install_hw_breakpoint(struct perf_event *bp) * * Atomic: we hold the counter->ctx->lock and we only handle variables * and registers local to this cpu. + * + * noinstr avoids getting hit with a breakpoint when the debug register is + * still active but we've already removed ourselves from bp_per_reg. */ -void arch_uninstall_hw_breakpoint(struct perf_event *bp) +void noinstr arch_uninstall_hw_breakpoint(struct perf_event *bp) { struct arch_hw_breakpoint *info = counter_arch_bp(bp); unsigned long dr7;