The patch below does not apply to the 4.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From eec13b42d41b0f3339dcf0c4da43734427c68620 Mon Sep 17 00:00:00 2001

From: Will Deacon will@kernel.org Date: Thu, 18 Jun 2020 11:16:45 +0100 Subject: [PATCH] ARM: 8986/1: hw_breakpoint: Don't invoke overflow handler on uaccess watchpoints

Unprivileged memory accesses generated by the so-called "translated" instructions (e.g. LDRT) in kernel mode can cause user watchpoints to fire unexpectedly. In such cases, the hw_breakpoint logic will invoke the user overflow handler which will typically raise a SIGTRAP back to the current task. This is futile when returning back to the kernel because (a) the signal won't have been delivered and (b) userspace can't handle the thing anyway.

Avoid invoking the user overflow handler for watchpoints triggered by kernel uaccess routines, and instead single-step over the faulting instruction as we would if no overflow handler had been installed.

Cc: stable@vger.kernel.org Fixes: f81ef4a920c8 ("ARM: 6356/1: hw-breakpoint: add ARM backend for the hw-breakpoint framework") Reported-by: Luis Machado luis.machado@linaro.org Tested-by: Luis Machado luis.machado@linaro.org Signed-off-by: Will Deacon will@kernel.org Signed-off-by: Russell King rmk+kernel@armlinux.org.uk

diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c index 02ca7adf5375..7fff88e61252 100644 --- a/arch/arm/kernel/hw_breakpoint.c +++ b/arch/arm/kernel/hw_breakpoint.c @@ -683,6 +683,12 @@ static void disable_single_step(struct perf_event *bp) arch_install_hw_breakpoint(bp); }

+static int watchpoint_fault_on_uaccess(struct pt_regs *regs, + struct arch_hw_breakpoint *info) +{ + return !user_mode(regs) && info->ctrl.privilege == ARM_BREAKPOINT_USER; +} + static void watchpoint_handler(unsigned long addr, unsigned int fsr, struct pt_regs *regs) { @@ -742,16 +748,27 @@ static void watchpoint_handler(unsigned long addr, unsigned int fsr, }

pr_debug("watchpoint fired: address = 0x%x\n", info->trigger); + + /* + * If we triggered a user watchpoint from a uaccess routine, + * then handle the stepping ourselves since userspace really + * can't help us with this. + */ + if (watchpoint_fault_on_uaccess(regs, info)) + goto step; + perf_bp_event(wp, regs);

/* - * If no overflow handler is present, insert a temporary - * mismatch breakpoint so we can single-step over the - * watchpoint trigger. + * Defer stepping to the overflow handler if one is installed. + * Otherwise, insert a temporary mismatch breakpoint so that + * we can single-step over the watchpoint trigger. */ - if (is_default_overflow_handler(wp)) - enable_single_step(wp, instruction_pointer(regs)); + if (!is_default_overflow_handler(wp)) + goto unlock;

+step: + enable_single_step(wp, instruction_pointer(regs)); unlock: rcu_read_unlock(); }