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SubjectRe: [PATCH] ARC: MM: fix UB and kernel resourse leak in do_page_fault
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On 11/7/18 4:12 AM, Eugeniy Paltsev wrote:
> Commit 15773ae938d8 ("signal/arc: Use force_sig_fault where
> appropriate") adds undefined behaviour and kernel resource leak
> to userspace in ARC do_page_fault() implementation.
>
> This happens because we don't initialize si_code variable after we
> switch to force_sig_fault using. si_code (as a part of siginfo_t
> structure) was previously initialized by clear_siginfo(&info) call
> which was removed.
>
> Undefined behaviour path:
> -------------------->8---------------------------
> }}} a/arch/arc/mm/fault.c
> !! -67,6 +67,7 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
> struct task_struct *tsk = current;
> struct mm_struct *mm = tsk->mm;
> + /// >>> si_code - uninitialized
> int si_code;
> int ret;
> vm_fault_t fault;
> int write = regs->ecr_cause & ECR_C_PROTV_STORE; /* ST/EX */
> !! -81,8 +82,10 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
> * only copy the information from the master page table,
> * nothing more.
> */
> + /// >>> take true branch
> if (address >= VMALLOC_START) {
> ret = handle_kernel_vaddr_fault(address);
> + /// >>> take true branch
> if (unlikely(ret))

While the fix is legit, are you sure this code path was taken. VMALLOC_START is
memory for kernel modules and/or vmalloc. So the fault was induced in kernel space
in which case user_mode)r) will not be true.


> + /// >>> jump to label "bad_area_nosemaphore"
> goto bad_area_nosemaphore;
> else
> !! -193,10 +196,13 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
> bad_area:
> up_read(&mm->mmap_sem);
>
> + /// >>> reach label "bad_area_nosemaphore"
> bad_area_nosemaphore:
> /* User mode accesses just cause a SIGSEGV */
> + /// >>> take true branch
> if (user_mode(regs)) {
> tsk->thread.fault_address = address;
> + /// >>> Ooops:
> + /// >>> use uninitialized value "si_code"
> + /// >>> when calling "force_sig_fault"
> force_sig_fault(SIGSEGV, si_code, (void __user *)address, tsk);
> return;
> }
> -------------------->8---------------------------
>
> Fixes: 15773ae938d8 ("signal/arc: Use force_sig_fault where appropriate")
> Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>

Added to for-curr after trimming the changelog

Thx,
-Vineet

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