Re: [PATCH] coredump: fix crash when umh is disabled

[Greg KH]

On Thu, Apr 16, 2020 at 04:28:59PM +0000, Luis R. Rodriguez wrote:
> From: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> 
> Commit 64e90a8acb859 ("Introduce STATIC_USERMODEHELPER to mediate
> call_usermodehelper()") added the optiont to disable all
> call_usermodehelper() calls by setting STATIC_USERMODEHELPER_PATH to
> an empty string. When this is done, and crashdump is triggered, it
> will crash on null pointer dereference, since we make assumptions
> over what call_usermodehelper_exec() did.
> 
> This has been reported by Sergey when one triggers a a coredump
> with the following configuration:
> 
> ```
> CONFIG_STATIC_USERMODEHELPER=y
> CONFIG_STATIC_USERMODEHELPER_PATH=""
> kernel.core_pattern = |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %e
> ```
> 
> The way disabling the umh was designed was that call_usermodehelper_exec()
> would just return early, without an error. But coredump assumes
> certain variables are set up for us when this happens, and calls
> ile_start_write(cprm.file) with a NULL file.
> 
> [    2.819676] BUG: kernel NULL pointer dereference, address: 0000000000000020
> [    2.819859] #PF: supervisor read access in kernel mode
> [    2.820035] #PF: error_code(0x0000) - not-present page
> [    2.820188] PGD 0 P4D 0
> [    2.820305] Oops: 0000 [#1] SMP PTI
> [    2.820436] CPU: 2 PID: 89 Comm: a Not tainted 5.7.0-rc1+ #7
> [    2.820680] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190711_202441-buildvm-armv7-10.arm.fedoraproject.org-2.fc31 04/01/2014
> [    2.821150] RIP: 0010:do_coredump+0xd80/0x1060
> [    2.821385] Code: e8 95 11 ed ff 48 c7 c6 cc a7 b4 81 48 8d bd 28 ff
> ff ff 89 c2 e8 70 f1 ff ff 41 89 c2 85 c0 0f 84 72 f7 ff ff e9 b4 fe ff
> ff <48> 8b 57 20 0f b7 02 66 25 00 f0 66 3d 00 8
> 0 0f 84 9c 01 00 00 44
> [    2.822014] RSP: 0000:ffffc9000029bcb8 EFLAGS: 00010246
> [    2.822339] RAX: 0000000000000000 RBX: ffff88803f860000 RCX: 000000000000000a
> [    2.822746] RDX: 0000000000000009 RSI: 0000000000000282 RDI: 0000000000000000
> [    2.823141] RBP: ffffc9000029bde8 R08: 0000000000000000 R09: ffffc9000029bc00
> [    2.823508] R10: 0000000000000001 R11: ffff88803dec90be R12: ffffffff81c39da0
> [    2.823902] R13: ffff88803de84400 R14: 0000000000000000 R15: 0000000000000000
> [    2.824285] FS:  00007fee08183540(0000) GS:ffff88803e480000(0000) knlGS:0000000000000000
> [    2.824767] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    2.825111] CR2: 0000000000000020 CR3: 000000003f856005 CR4: 0000000000060ea0
> [    2.825479] Call Trace:
> [    2.825790]  get_signal+0x11e/0x720
> [    2.826087]  do_signal+0x1d/0x670
> [    2.826361]  ? force_sig_info_to_task+0xc1/0xf0
> [    2.826691]  ? force_sig_fault+0x3c/0x40
> [    2.826996]  ? do_trap+0xc9/0x100
> [    2.827179]  exit_to_usermode_loop+0x49/0x90
> [    2.827359]  prepare_exit_to_usermode+0x77/0xb0
> [    2.827559]  ? invalid_op+0xa/0x30
> [    2.827747]  ret_from_intr+0x20/0x20
> [    2.827921] RIP: 0033:0x55e2c76d2129
> [    2.828107] Code: 2d ff ff ff e8 68 ff ff ff 5d c6 05 18 2f 00 00 01
> c3 0f 1f 80 00 00 00 00 c3 0f 1f 80 00 00 00 00 e9 7b ff ff ff 55 48 89
> e5 <0f> 0b b8 00 00 00 00 5d c3 66 2e 0f 1f 84 0
> 0 00 00 00 00 0f 1f 40
> [    2.828603] RSP: 002b:00007fffeba5e080 EFLAGS: 00010246
> [    2.828801] RAX: 000055e2c76d2125 RBX: 0000000000000000 RCX: 00007fee0817c718
> [    2.829034] RDX: 00007fffeba5e188 RSI: 00007fffeba5e178 RDI: 0000000000000001
> [    2.829257] RBP: 00007fffeba5e080 R08: 0000000000000000 R09: 00007fee08193c00
> [    2.829482] R10: 0000000000000009 R11: 0000000000000000 R12: 000055e2c76d2040
> [    2.829727] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> [    2.829964] CR2: 0000000000000020
> [    2.830149] ---[ end trace ceed83d8c68a1bf1 ]---
> ```
> 
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.11+
> Fixes: 64e90a8acb859 ("Introduce STATIC_USERMODEHELPER to mediate call_usermodehelper()")

Nit, you don't need so many digits, it should be:
Fixes: 64e90a8acb85 ("Introduce STATIC_USERMODEHELPER to mediate call_usermodehelper()")

> BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=199795
> Reported-by: Tony Vroon <chainsaw@xxxxxxxxxx>
> Reported-by: Sergey Kvachonok <ravenexp@xxxxxxxxx>
> Tested-by: Sergei Trofimovich <slyfox@xxxxxxxxxx>
> Signed-off-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> ---
>  fs/coredump.c | 8 ++++++++
>  kernel/umh.c  | 5 +++++
>  2 files changed, 13 insertions(+)

Anyway, I can take this in my driver core tree if no one else objects.

thanks,

greg k-h