Re: [PATCH v3 01/12] powerpc: mm/fault: Fix kfence page fault reporting
From: Michael Ellerman <mpe@ellerman.id.au>
Date: 2024-10-22 02:42:33
Also in:
linux-mm, lkml
Hi Ritesh, "Ritesh Harjani (IBM)" [off-list ref] writes:
copy_from_kernel_nofault() can be called when doing read of /proc/kcore. /proc/kcore can have some unmapped kfence objects which when read via copy_from_kernel_nofault() can cause page faults. Since *_nofault() functions define their own fixup table for handling fault, use that instead of asking kfence to handle such faults. Hence we search the exception tables for the nip which generated the fault. If there is an entry then we let the fixup table handler handle the page fault by returning an error from within ___do_page_fault(). This can be easily triggered if someone tries to do dd from /proc/kcore. dd if=/proc/kcore of=/dev/null bs=1M <some example false negatives> =============================== BUG: KFENCE: invalid read in copy_from_kernel_nofault+0xb0/0x1c8 Invalid read at 0x000000004f749d2e: copy_from_kernel_nofault+0xb0/0x1c8 0xc0000000057f7950 read_kcore_iter+0x41c/0x9ac proc_reg_read_iter+0xe4/0x16c vfs_read+0x2e4/0x3b0 ksys_read+0x88/0x154 system_call_exception+0x124/0x340 system_call_common+0x160/0x2c4
I haven't been able to reproduce this. Can you give some more details on the exact machine/kernel-config/setup where you saw this? cheers