Messages in this thread | | | Date | Thu, 16 Apr 2020 16:29:58 +0000 | From | Luis Chamberlain <> | Subject | Re: [PATCH] umh: always return error when helper was not called |
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On Thu, Apr 16, 2020 at 08:03:16AM +0100, Sergei Trofimovich wrote: > On Thu, 16 Apr 2020 05:11:27 +0000 > Luis Chamberlain <mcgrof@kernel.org> wrote: > > > 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. > > I agree changing existing API semantics is dangerous. > Maybe tweaking a docstring to call_usermodehelper_exec() would be helpful. > Something like: > > --- a/kernel/umh.c > +++ b/kernel/umh.c > @@ -544,6 +544,11 @@ EXPORT_SYMBOL_GPL(fork_usermode_blob); > * Runs a user-space application. The application is started > * asynchronously if wait is not set, and runs as a child of system workqueues. > * (ie. it runs with full root capabilities and optimized affinity). > + * > + * Note: successful return value does not guarantee helper was called at all. > + * You can't rely on sub_info->{init,cleanup} being called even for UMH_WAIT_* > + * wait modes as STATIC_USERMODEHELPER_PATH="" turns all helpers > + * into a successful no-op. > */ > int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait) > { > > > Can you try this patch instead? > > Gave it a light crash-test locally. Works fine! > > Tested-by: Sergei Trofimovich <slyfox@gentoo.org>
Thanks, I'll send a follow up.
Luis
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