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SubjectRe: general protection fault in perf_misc_flags
On Wed, Sep 30, 2020 at 6:17 PM Borislav Petkov <bp@alien8.de> wrote:
>
> Hi,
>
> one more thing I just spotted. The default install of syzkaller here
> runs the guest with this on the kernel command line:
>
> 2020/09/30 17:56:18 running command: qemu-system-x86_64 []string{"-m", "2048",
> "-smp", "2", "-display", ... "-append", "earlyprintk=serial oops=panic ...
> nmi_watchdog=panic panic_on_warn=1 panic=1 ftrace_dump_on_oops=orig_cpu rodata=n
> ^^^^^^^^^^
>
> which basically leaves guest kernel's memory RW and it gets caught
> immediately on vm boot by CONFIG_DEBUG_WX.
>
> This pretty much explains why kernel text can get corrupted with a stray
> pointer write or so. So what's the use case for rodata=n?
>
> [ 2.478136] Kernel memory protection disabled.
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


Ha!

Here is the answer:
https://github.com/google/syzkaller/blob/master/tools/create-gce-image.sh#L189

# rodata=n: mark_rodata_ro becomes very slow with KASAN (lots of PGDs)

I have some vague memory that there was some debug double checking
that pages are indeed read-only and that debug check was slow, but it
was always executed without rodata=n.

> [ 2.478689] x86/mm: Checking user space page tables
> [ 2.550163] ------------[ cut here ]------------
> [ 2.550736] x86/mm: Found insecure W+X mapping at address entry_SYSCALL_64+0x0/0x29
> [ 2.551612] WARNING: CPU: 1 PID: 1 at arch/x86/mm/dump_pagetables.c:246 note_page+0x81f/0x13a0
> [ 2.552577] Kernel panic - not syncing: panic_on_warn set ...
> [ 2.553240] CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.9.0-rc7+ #5
> [ 2.553953] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-1 04/01/2014
> [ 2.554922] Call Trace:
> [ 2.555233] dump_stack+0x9c/0xcf
> [ 2.555633] panic+0x250/0x5a0
> [ 2.556004] ? __warn_printk+0xf8/0xf8
> [ 2.556450] ? console_trylock+0xb0/0xb0
> [ 2.556914] ? __warn.cold+0x5/0x44
> [ 2.557332] ? note_page+0x81f/0x13a0
> [ 2.557768] __warn.cold+0x20/0x44
> [ 2.558176] ? note_page+0x81f/0x13a0
> [ 2.558641] report_bug+0x168/0x1b0
> [ 2.559059] handle_bug+0x3c/0x60
> [ 2.559458] exc_invalid_op+0x14/0x40
> [ 2.559894] asm_exc_invalid_op+0x12/0x20
> [ 2.560368] RIP: 0010:note_page+0x81f/0x13a0
> [ 2.560870] Code: 26 00 80 3d 9a d0 7f 02 00 0f 85 82 f9 ff ff e8 47 3c 26 00 4c 89 e6 48 c7 c7 40 aff
> [ 2.562951] RSP: 0000:ffff88800e9f7a90 EFLAGS: 00010282
> [ 2.563554] RAX: 0000000000000000 RBX: ffff88800e9f7e00 RCX: 0000000000000000
> [ 2.564361] RDX: ffff88800e9edb80 RSI: 0000000000000004 RDI: ffffed1001d3ef44
> [ 2.565167] RBP: 0000000000000200 R08: 0000000000000001 R09: 0000000000000003
> [ 2.565973] R10: ffffed1001d3eefd R11: 0000000000000001 R12: ffffffff96e00000
> [ 2.566780] R13: 00000000000001e3 R14: 0000000000000000 R15: ffff88800e9f7e58
> [ 2.567587] ? __kprobes_text_end+0xb3598/0xb3598
> [ 2.568122] ? __entry_text_end+0x1fea85/0x1fea85
> [ 2.568754] ? __entry_text_end+0x1fea85/0x1fea85
> [ 2.569288] ? __entry_text_end+0x1fea85/0x1fea85
> [ 2.569821] ptdump_hole+0x61/0x90
> [ 2.570212] ? ptdump_pte_entry+0x100/0x100
> [ 2.570712] walk_pgd_range+0xdb8/0x15f0
> [ 2.571178] walk_page_range_novma+0xd9/0x140
> [ 2.571689] ? walk_page_range+0x2b0/0x2b0
> [ 2.572171] ? console_unlock+0x58f/0xb10
> [ 2.572644] ptdump_walk_pgd+0xcd/0x180
> [ 2.573099] ptdump_walk_pgd_level_core+0x13c/0x1b0
> [ 2.573663] ? effective_prot+0xb0/0xb0
> [ 2.574117] ? vprintk_emit+0x214/0x380
> [ 2.574601] ? ptdump_walk_pgd_level_core+0x1b0/0x1b0
> [ 2.575186] ? memtype_copy_nth_element+0x1a0/0x1a0
> [ 2.575752] ? __kprobes_text_end+0xb3598/0xb3598
> [ 2.576300] ? pti_user_pagetable_walk_pmd+0x130/0x460
> [ 2.576894] ? __kprobes_text_end+0xb3598/0xb3598
> [ 2.577441] ? __kprobes_text_end+0xb3598/0xb3598
> [ 2.577988] ? __kprobes_text_end+0xb3598/0xb3598
> [ 2.578564] ? rest_init+0xdd/0xdd
> [ 2.578972] ptdump_walk_user_pgd_level_checkwx.cold+0x31/0x36
> [ 2.579640] pti_finalize+0x7b/0x170
> [ 2.580066] kernel_init+0x5b/0x183
> [ 2.580484] ret_from_fork+0x22/0x30
> [ 2.581010] Dumping ftrace buffer:
> [ 2.581456] (ftrace buffer empty)
> ffffffffbfffffff)
> [ 2.583137] Rebooting in 1 seconds..
> 2020/09/30 17:56:23 failed to create instance: failed to read from qemu: EOF
>
> --
> Regards/Gruss,
> Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette

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