• Luis Chamberlain's avatar
    coredump: fix crash when umh is disabled · 3740d93e
    Luis Chamberlain authored
    Commit 64e90a8a ("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@vger.kernel.org> # v4.11+
    Fixes: 64e90a8a ("Introduce STATIC_USERMODEHELPER to mediate call_usermodehelper()")
    BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=199795Reported-by: default avatarTony Vroon <chainsaw@gentoo.org>
    Reported-by: default avatarSergey Kvachonok <ravenexp@gmail.com>
    Tested-by: default avatarSergei Trofimovich <slyfox@gentoo.org>
    Signed-off-by: default avatarLuis Chamberlain <mcgrof@kernel.org>
    Link: https://lore.kernel.org/r/20200416162859.26518-1-mcgrof@kernel.orgSigned-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    3740d93e
umh.c 19.3 KB