Messages in this thread | | | Date | Wed, 16 Mar 2016 10:27:22 +0000 | From | James Morse <> | Subject | Re: [PATCH v11 3/9] arm64: add copy_to/from_user to kprobes blacklist |
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Hi Pratyush,
On 16/03/16 05:43, Pratyush Anand wrote: > On 15/03/2016:06:47:52 PM, James Morse wrote: >> If I understand this correctly - you can't kprobe these ldr/str instructions >> as the fault handler wouldn't find kprobe's out-of line version of the >> instruction in the exception table... but why only these two functions? (for >> library functions, we also have clear_user() and copy_in_user()...) > > May be not clear_user() because those are inlined, but may be __clear_user().
You're right - the other library functions in that same directory is what I meant..
>> Is it feasible to search the exception table at runtime instead? If an >> address-to-be-kprobed appears in the list, we know it could generate exceptions, >> so we should report that we can't probe this address. That would catch all of >> the library functions, all the places uaccess.h was inlined, and anything new >> that gets invented in the future. > > Sorry, probably I could not get it. How can an inlined addresses range be placed > in exception table or any other code area.
Ah, not a section or code area, sorry I wasn't clear:
When a fault happens in the kernel, the fault handler (/arch/arm64/mm/fault.c:do_page_fault()) calls search_exception_tables(regs->pc) to see if the faulting address has a 'fixup' registered. If it does, the fixup causes -EFAULT to be returned, if not it ends up in die().
The horrible block of assembler in arch/arm64/include/asm/uaccess.h:__get_user_asm() adds the address of the instruction that is allowed to fault to the __ex_table section: > .section __ex_table,"a" > .align 3 > .quad 1b, 3b > .previous
Here 1b is the address of the instruction that can fault, and 3b is the fixup that moves -EFAULT into the return value.
This works for get_user() and friends which are inlined all over the kernel. It even works for modules, as there is an exception table for each module which is searched by kernel/module.c:search_module_extables().
This list of addresses that can fault already exists, there is even an API function to check for a given address. Grabbing the nearest vmlinux, there are ~1300 entries in the __ex_table section, this patch blacklists two of them, using search_exception_tables() obviously blacklists them all.
I've had a quick look at x86 and sparc, it looks like they allowed probed instructions to fault, do_page_fault()->kprobes_fault()->kprobe_fault_handler() - which uses the original probed address with search_exception_tables() to find and run the fixup. I doubt this is needed in an initial version of kprobes, (maybe its later in this series - I haven't read all the way through it yet).
Thanks,
James
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