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SubjectUBSAN: array-index-out-of-bounds in arch_uprobe_analyze_insn

syzbot found the following issue on:

HEAD commit: 325d0eab Merge branch 'akpm' (patches from Andrew)
git tree: upstream
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 10.1.0-syz 20200507

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:

UBSAN: array-index-out-of-bounds in arch/x86/kernel/uprobes.c:263:56
index 4 is out of range for type 'insn_byte_t [4]'
CPU: 1 PID: 22315 Comm: syz-executor.4 Not tainted 5.9.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x198/0x1fd lib/dump_stack.c:118
ubsan_epilogue+0xb/0x5a lib/ubsan.c:148
__ubsan_handle_out_of_bounds.cold+0x62/0x6c lib/ubsan.c:356
is_prefix_bad arch/x86/kernel/uprobes.c:263 [inline]
uprobe_init_insn arch/x86/kernel/uprobes.c:286 [inline]
arch_uprobe_analyze_insn+0x8f4/0xa40 arch/x86/kernel/uprobes.c:856
prepare_uprobe kernel/events/uprobes.c:860 [inline]
install_breakpoint.isra.0+0x6c4/0x7c0 kernel/events/uprobes.c:903
uprobe_mmap+0x5ec/0x1080 kernel/events/uprobes.c:1394
mmap_region+0x5cf/0x1780 mm/mmap.c:1839
do_mmap+0xcf9/0x11d0 mm/mmap.c:1545
vm_mmap_pgoff+0x195/0x200 mm/util.c:506
ksys_mmap_pgoff+0x444/0x580 mm/mmap.c:1596
do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
RIP: 0033:0x45d5f9
Code: 5d b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 2b b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f8e919bcc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000009
RAX: ffffffffffffffda RBX: 0000000000020f80 RCX: 000000000045d5f9
RDX: 0000000000000000 RSI: 0000000000003000 RDI: 0000000020007000
RBP: 000000000118cf98 R08: 0000000000000003 R09: 0000000000000000
R10: 0000000000000412 R11: 0000000000000246 R12: 000000000118cf4c
R13: 00007ffe8fbd4c5f R14: 00007f8e919bd9c0 R15: 000000000118cf4c

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