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powerpc/watchpoint: Fix exception handling for CONFIG_HAVE_HW_BREAKPOINT=N 

On powerpc, ptrace watchpoint works in one-shot mode. i.e. kernel
disables event every time it fires and user has to re-enable it.
Also, in case of ptrace watchpoint, kernel notifies ptrace user
before executing instruction.

With CONFIG_HAVE_HW_BREAKPOINT=N, kernel is missing to disable
ptrace event and thus it's causing infinite loop of exceptions.
This is especially harmful when user watches on a data which is
also read/written by kernel, eg syscall parameters. In such case,
infinite exceptions happens in kernel mode which causes soft-lockup.

Fixes: 9422de3e95 ("powerpc: Hardware breakpoints rewrite to handle non DABR breakpoint registers")
Reported-by: Pedro Miraglia Franco de Carvalho <pedromfc@linux.ibm.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200902042945.129369-6-ravi.bangoria@linux.ibm.com
This commit is contained in:
Ravi Bangoria 2020-09-02 09:59:42 +05:30 committed by Michael Ellerman
parent edc8dd99b2
commit 5b905d7798
3 changed files with 54 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
arch/powerpc/include/asm/hw_breakpoint.h
View File

@ -18,6 +18,7 @@ struct arch_hw_breakpoint {
u16 type;
u16 len; /* length of the target data symbol */
u16 hw_len; /* length programmed in hw */
u8 flags;
};
/* Note: Don't change the first 6 bits below as they are in the same order
@ -37,6 +38,8 @@ struct arch_hw_breakpoint {
#define HW_BRK_TYPE_PRIV_ALL (HW_BRK_TYPE_USER | HW_BRK_TYPE_KERNEL | \
HW_BRK_TYPE_HYP)
#define HW_BRK_FLAG_DISABLED 0x1
/* Minimum granularity */
#ifdef CONFIG_PPC_8xx
#define HW_BREAKPOINT_SIZE 0x4

48
arch/powerpc/kernel/process.c
View File

@ -642,6 +642,44 @@ void do_send_trap(struct pt_regs *regs, unsigned long address,
(void __user *)address);
}
#else /* !CONFIG_PPC_ADV_DEBUG_REGS */
static void do_break_handler(struct pt_regs *regs)
{
struct arch_hw_breakpoint null_brk = {0};
struct arch_hw_breakpoint *info;
struct ppc_inst instr = ppc_inst(0);
int type = 0;
int size = 0;
unsigned long ea;
int i;
/*
* If underneath hw supports only one watchpoint, we know it
* caused exception. 8xx also falls into this category.
*/
if (nr_wp_slots() == 1) {
__set_breakpoint(0, &null_brk);
current->thread.hw_brk[0] = null_brk;
current->thread.hw_brk[0].flags |= HW_BRK_FLAG_DISABLED;
return;
}
/* Otherwise findout which DAWR caused exception and disable it. */
wp_get_instr_detail(regs, &instr, &type, &size, &ea);
for (i = 0; i < nr_wp_slots(); i++) {
info = &current->thread.hw_brk[i];
if (!info->address)
continue;
if (wp_check_constraints(regs, instr, ea, type, size, info)) {
__set_breakpoint(i, &null_brk);
current->thread.hw_brk[i] = null_brk;
current->thread.hw_brk[i].flags |= HW_BRK_FLAG_DISABLED;
}
}
}
void do_break (struct pt_regs *regs, unsigned long address,
unsigned long error_code)
{
@ -653,6 +691,16 @@ void do_break (struct pt_regs *regs, unsigned long address,
if (debugger_break_match(regs))
return;
/*
* We reach here only when watchpoint exception is generated by ptrace
* event (or hw is buggy!). Now if CONFIG_HAVE_HW_BREAKPOINT is set,
* watchpoint is already handled by hw_breakpoint_handler() so we don't
* have to do anything. But when CONFIG_HAVE_HW_BREAKPOINT is not set,
* we need to manually handle the watchpoint here.
*/
if (!IS_ENABLED(CONFIG_HAVE_HW_BREAKPOINT))
do_break_handler(regs);
/* Deliver the signal to userspace */
force_sig_fault(SIGTRAP, TRAP_HWBKPT, (void __user *)address);
}

4
arch/powerpc/kernel/ptrace/ptrace-noadv.c
View File

@ -286,11 +286,13 @@ long ppc_del_hwdebug(struct task_struct *child, long data)
}
return ret;
#else /* CONFIG_HAVE_HW_BREAKPOINT */
if (child->thread.hw_brk[data - 1].address == 0)
if (!(child->thread.hw_brk[data - 1].flags & HW_BRK_FLAG_DISABLED) &&
child->thread.hw_brk[data - 1].address == 0)
return -ENOENT;
child->thread.hw_brk[data - 1].address = 0;
child->thread.hw_brk[data - 1].type = 0;
child->thread.hw_brk[data - 1].flags = 0;
#endif /* CONFIG_HAVE_HW_BREAKPOINT */
return 0;