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