Messages in this thread Patch in this message | | | Date | Thu, 16 Apr 2020 05:11:27 +0000 | From | Luis Chamberlain <> | Subject | Re: [PATCH] umh: always return error when helper was not called |
| |
Sergei, first, thanks for your patch and bug report!!
On Wed, Apr 15, 2020 at 07:59:40AM +0100, Sergei Trofimovich wrote: > Before this change on a system with the following setup crashed kernel: > > ``` > CONFIG_STATIC_USERMODEHELPER=y > CONFIG_STATIC_USERMODEHELPER_PATH="" > kernel.core_pattern = |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %e > ```
Let us backtrack. The combination of:
CONFIG_STATIC_USERMODEHELPER=y CONFIG_STATIC_USERMODEHELPER_PATH=""
is documented on the kconfig files for when you *want to disable all usermode helper programs.
> The crash happens when a core dump is attempted: > > ``` > [ 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 ]--- > ``` > > Here is the sequence of events why it happens: > fs/coredump.c:do_coredump(): > 1. create 'coredump_params = { .file = NULL }' > 2. detect pipe mode > 3. `call_usermodehelper_setup(..., umh_pipe_setup, ...)` > 4. `call_usermodehelper_exec()` > 5. (if both succeeded) `file_start_write(cprm.file);` > > Here crash happens at [5.] as `cprm.file` is still NULL. > > Normally it works because `fs/coredump.c:umh_pipe_setup()` is called > successfully and populates `.file` field (or returns the error): > > ``` > static int umh_pipe_setup(struct subprocess_info *info, struct cred *new) > { > //... > struct coredump_params *cp = (struct coredump_params *)info->data; > // ... > cp->file = files[1]; > // ... > } > ``` > > But in our case neither happens because `kernel/umh.c:call_usermodehelper_exec()` > has a special case: > > ``` > int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait) > { > int retval = 0; > // ... > /* > * If there is no binary for us to call, then just return and get out of > * here. This allows us to set STATIC_USERMODEHELPER_PATH to "" and > * disable all call_usermodehelper() calls. > */ > if (strlen(sub_info->path) == 0) > goto out; > ... > out: > // ... > return retval; > > ``` > > This breaks assumption of `do_coredump()`: "either helper was called successfully > and created a file to dump core to or it failed". > > This change converts this special case to `-EPERM` error. > > This way we notify user that helper call was not successful > and don't attempt to act on uninitialized `.file` field. > > User gets `"Core dump to |%s pipe failed\n` dmesg entry. > > Reported-by: Sergey Kvachonok <ravenexp@gmail.com> > Reported-by: Tony Vroon <chainsaw@gentoo.org> > Bug: https://bugzilla.kernel.org/show_bug.cgi?id=199795 > Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org> > CC: Luis Chamberlain <mcgrof@kernel.org> > --- > kernel/umh.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/kernel/umh.c b/kernel/umh.c > index 7f255b5a8845..66b02634a9ba 100644 > --- a/kernel/umh.c > +++ b/kernel/umh.c > @@ -565,8 +565,10 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait) > * here. This allows us to set STATIC_USERMODEHELPER_PATH to "" and > * disable all call_usermodehelper() calls. > */ > - if (strlen(sub_info->path) == 0) > + if (strlen(sub_info->path) == 0) { > + retval = -EPERM; > goto out; > + } > > /* > * Set the completion pointer only if there is a waiter.
Stakeholders with this setup likely already are relying on the fact that we don't return -EPERM. Your change to return -EPERM may fix the crash you are seeing, but it may also break existing userspace, granted for a crashdump that may not matter much, however I agree it is stupid to crash on a crash :)
Anyway, I don't think its a good idea to return -EPERM unless stakeholders really find a strong reason to change old behaviour.
Can you try this patch instead?
diff --git a/fs/coredump.c b/fs/coredump.c index f8296a82d01d..6957d513685e 100644 --- a/fs/coredump.c +++ b/fs/coredump.c @@ -786,6 +786,15 @@ void do_coredump(const kernel_siginfo_t *siginfo) if (displaced) put_files_struct(displaced); if (!dump_interrupted()) { + /* + * umh disabled with CONFIG_STATIC_USERMODEHELPER_PATH="" would + * have this set to NULL. + */ + if (!cprm.file) { + printk(KERN_INFO "Core dump to |%s disabled\n", + cn.corename); + goto close_fail; + } file_start_write(cprm.file); core_dumped = binfmt->core_dump(&cprm); file_end_write(cprm.file);
| |