Messages in this thread Patch in this message | | | From | Thomas Gleixner <> | Subject | Re: [PATCH v2] x86/dumpstack: Fix misleading instruction pointer error message | Date | Mon, 16 Nov 2020 23:01:03 +0100 |
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On Tue, Nov 03 2020 at 19:20, Borislav Petkov wrote: > On Tue, Nov 03, 2020 at 07:11:15PM +0100, Oleg Nesterov wrote: >> > I'm thinking copy_code() should not use copy_from_user_nmi() if former >> > can be called in non-atomic context too.
While copy_from_user_nmi() is named that way, it can be invoked from other contexts as well. See the comment inside.
>> I understand, but why do you think this makes sense? > > Because the copy_from_user_nmi()'s name tells me that it is at least > supposed to be called in atomic context. At least this is how I > understand it. And in atomic context regs is supposed to belong to > current, right?
Whatever context you are in current can only read it's own user space obviously.
AFAICT even before the change I did, show_opcodes() did not care and just either dumped what was available at regs->ip in the current tasks user space mapping or faulted.
Fix below.
Thanks,
tglx --- Subject: x86/dumpstack: Don't try to access user space code of other tasks From: Thomas Gleixner <tglx@linutronix.de> Date: Mon, 16 Nov 2020 22:26:52 +0100
sysrq-t ends up invoking show_opcodes() for each task which tries to access the user space code of other processes which is obviously bogus.
It either manages to dump where the foreign tasks regs->ip points to in currents mapping or triggers a pagefault and prints "Code: Bad RIP value.". Both is just wrong.
Add a safeguard in copy_code() and check whether the @regs pointer matches currents pt_regs. If not, do not even try to access it.
While at it, add commentry why using copy_from_user_nmi() is safe in copy_code() even if the function name suggests otherwise.
Reported-by: Mark Mossberg <mark.mossberg@gmail.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> --- arch/x86/kernel/dumpstack.c | 23 +++++++++++++++++++---- 1 file changed, 19 insertions(+), 4 deletions(-)
--- a/arch/x86/kernel/dumpstack.c +++ b/arch/x86/kernel/dumpstack.c @@ -78,6 +78,9 @@ static int copy_code(struct pt_regs *reg if (!user_mode(regs)) return copy_from_kernel_nofault(buf, (u8 *)src, nbytes); + /* The user space code from other tasks cannot be accessed. */ + if (regs != task_pt_regs(current)) + return -EPERM; /* * Make sure userspace isn't trying to trick us into dumping kernel * memory by pointing the userspace instruction pointer at it. @@ -85,6 +88,12 @@ static int copy_code(struct pt_regs *reg if (__chk_range_not_ok(src, nbytes, TASK_SIZE_MAX)) return -EINVAL; + /* + * Even if named copy_from_user_nmi() this can be invoked from + * other contexts and will not try to resolve a pagefault, which is + * the correct thing to do here as this code can be called from any + * context. + */ return copy_from_user_nmi(buf, (void __user *)src, nbytes); } @@ -115,13 +124,19 @@ void show_opcodes(struct pt_regs *regs, u8 opcodes[OPCODE_BUFSIZE]; unsigned long prologue = regs->ip - PROLOGUE_SIZE; - if (copy_code(regs, opcodes, prologue, sizeof(opcodes))) { - printk("%sCode: Unable to access opcode bytes at RIP 0x%lx.\n", - loglvl, prologue); - } else { + switch (copy_code(regs, opcodes, prologue, sizeof(opcodes))) { + case 0: printk("%sCode: %" __stringify(PROLOGUE_SIZE) "ph <%02x> %" __stringify(EPILOGUE_SIZE) "ph\n", loglvl, opcodes, opcodes[PROLOGUE_SIZE], opcodes + PROLOGUE_SIZE + 1); + break; + case -EPERM: + /* No access to the user space stack of other tasks. Ignore. */ + break; + default: + printk("%sCode: Unable to access opcode bytes at RIP 0x%lx.\n", + loglvl, prologue); + break; } }
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