| Message ID | 20220927093159.20056-1-matthew.ruffell@canonical.com |
|---|---|
| Headers | show |
| Series | LSM: Configuring Too Many LSMs Causes Kernel Panic on Boot | expand |
On 27.09.22 11:31, Matthew Ruffell wrote: > BugLink: https://bugs.launchpad.net/bugs/1987998 > > [Impact] > > The Ubuntu kernel carries an out of tree patchet, known as > "LSM: Module stacking for AppArmor" upstream, to enable stackable LSMs for > containers. The revision the Ubuntu kernel carries is an older one, from 2020, > and has some slight divergences from the latest revision in development. > > One such divergence, is support for Landlock as a stackable LSM. When the > stackable LSM patchset was applied, Landlock was still in development and not > mainlined yet, and wasn't present in the earlier revision of the > "LSM: Module stacking for AppArmor" patchset. Support for this was added by us. > > There was a minor omission made during enabling support for Landlock. The LSM > slot type was marked as LSMBLOB_NEEDED, when it should have been > LSMBLOB_NOT_NEEDED. > > Landlock itself does not provide any of the hooks that use a struct lsmblob, > such as secid_to_secctx, secctx_to_secid, inode_getsecid, cred_getsecid, > kernel_act_as task_getsecid_subj task_getsecid_obj and ipc_getsecid. > > When we set .slot = LSMBLOB_NEEDED, this indicates that we need an entry in > struct lsmblob, and we need to increment LSMBLOB_ENTRIES by one to fit the entry > into the secid array: > > #define LSMBLOB_ENTRIES ( \ > (IS_ENABLED(CONFIG_SECURITY_SELINUX) ? 1 : 0) + \ > (IS_ENABLED(CONFIG_SECURITY_SMACK) ? 1 : 0) + \ > (IS_ENABLED(CONFIG_SECURITY_APPARMOR) ? 1 : 0) + \ > (IS_ENABLED(CONFIG_BPF_LSM) ? 1 : 0)) > > struct lsmblob { > u32 secid[LSMBLOB_ENTRIES]; > }; > > Currently, we don't increment LSMBLOB_ENTRIES by one to make an entry for > Landlock, so for the Ubuntu kernel, we can fit a maximum of two entries, one for > Apparmor and one for bpf. > > If you try and configure three LSMs like so and reboot: > > GRUB_CMDLINE_LINUX_DEFAULT="lsm=landlock,bpf,apparmor" > > You will receive the following panic: > > LSM: Security Framework initializing > landlock: Up and running. > LSM support for eBPF active > Kernel panic - not syncing: security_add_hooks Too many LSMs registered. > CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.15.0-46-generic #49-Ubuntu > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014 > Call Trace: > <TASK> > show_stack+0x52/0x5c > dump_stack_lvl+0x4a/0x63 > dump_stack+0x10/0x16 > panic+0x149/0x321 > security_add_hooks+0x45/0x13a > apparmor_init+0x189/0x1ef > initialize_lsm+0x54/0x74 > ordered_lsm_init+0x379/0x392 > security_init+0x40/0x49 > start_kernel+0x466/0x4dc > x86_64_start_reservations+0x24/0x2a > x86_64_start_kernel+0xe4/0xef > secondary_startup_64_no_verify+0xc2/0xcb > </TASK> > ---[ end Kernel panic - not syncing: security_add_hooks Too many LSMs registered. ]--- > > There is a check added in security_add_hooks() that makes sure that you cannot > configure too many LSMs: > > if (lsmid->slot == LSMBLOB_NEEDED) { > if (lsm_slot >= LSMBLOB_ENTRIES) > panic("%s Too many LSMs registered.\n", __func__); > lsmid->slot = lsm_slot++; > init_debug("%s assigned lsmblob slot %d\n", lsmid->lsm, > lsmid->slot); > } > > A workaround is to enable no more than 2 LSMs until this is fixed. > > [Fix] > > If you read the following mailing list thread on linux-security-modules from > May 2021: > > https://lore.kernel.org/selinux/202105141224.942DE93@keescook/T/ > > It is explained that Landlock does not provide any of the hooks that use a > struct lsmblob, such as secid_to_secctx, secctx_to_secid, inode_getsecid, > cred_getsecid, kernel_act_as task_getsecid_subj task_getsecid_obj and > ipc_getsecid. > > I verified this with: > > ubuntu-jammy$ grep -Rin "secid_to_secctx" security/landlock/ > ubuntu-jammy$ grep -Rin "secctx_to_secid" security/landlock/ > ubuntu-jammy$ grep -Rin "inode_getsecid" security/landlock/ > ubuntu-jammy$ grep -Rin "cred_getsecid" security/landlock/ > ubuntu-jammy$ grep -Rin "kernel_act_as" security/landlock/ > ubuntu-jammy$ grep -Rin "task_getsecid_subj" security/landlock/ > ubuntu-jammy$ grep -Rin "task_getsecid_obj" security/landlock/ > ubuntu-jammy$ grep -Rin "ipc_getsecid" security/landlock/ > > The fix is to change Landlock from LSMBLOB_NEEDED to LSMBLOB_NOT_NEEDED. > > Due to the "LSM: Module stacking for AppArmor" patchset being 25 patches long, > it was impractical to revert just the below patch and reapply with the fix, due > to a large amount of conflicts: > > commit f17b27a2790e72198d2aaf45242453e5a9043049 > Author: Casey Schaufler <casey@schaufler-ca.com> > Date: Mon Aug 17 16:02:56 2020 -0700 > Subject: UBUNTU: SAUCE: LSM: Create and manage the lsmblob data structure. > Link: https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/jammy/commit/?id=f17b27a2790e72198d2aaf45242453e5a9043049 > > So instead, I wrote up a fix that just changes the Landlock LSM slots to follow > the latest upstream development, from V37 of the patchset: > > https://lore.kernel.org/selinux/20220628005611.13106-4-casey@schaufler-ca.com/ > > I refactored the landlock_lsmid struct to only be in one place, and to be marked > as extern from security/landlock/setup.h. > > [Testcase] > > Launch a Jammy or Kinetic VM. > > 1. Edit /etc/default/grub and append the following to GRUB_CMDLINE_LINUX_DEFAULT: > "lsm=landlock,bpf,apparmor" > 2. sudo update-grub > 3. reboot > > The system will panic on boot. > > If you install the test kernel from the following ppa: > > https://launchpad.net/~mruffell/+archive/ubuntu/sf343286-test > > Instead of a panic occurring, the kernel should display all LSMs initialising, > and continue to boot. > > [ 0.288224] LSM: Security Framework initializing > [ 0.289457] landlock: Up and running. > [ 0.290290] LSM support for eBPF active > [ 0.291189] AppArmor: AppArmor initialized > > [Where problems could occur] > > The risk of regression in changing Landlock from LSMBLOB_NEEDED to > LSMBLOB_NOT_NEEDED is low, due to Landlock not needing a slot in the secid array > in struct lsmblob in the first place. > > The refactor is minor and unlikely to introduce any issues with Landlock or its > security promises. > > I feel that simply fixing this small bug is less regression risk than reverting > the entire 25 patch patchset and applying the latest V37 upstream patchset, > which has undergone significant changes from mid 2020. I think its best we > consume the newer patchset once it makes its way into mainline in a future > kernel instead. > > If a regression were to occur, users could configure 2 LSMs instead of all 3, > or not enable Landlock. > > [Other Info] > > This patchset was originally NACKed by Stefan Bader on request of John > Johansen due to a minor misunderstanding on how LSMBLOB_NEEDED worked with > blob slot registration. > > ACK: > https://lists.ubuntu.com/archives/kernel-team/2022-August/132898.html > Applied: > https://lists.ubuntu.com/archives/kernel-team/2022-August/132887.html > NACK: > https://lists.ubuntu.com/archives/kernel-team/2022-September/132965.html > Revert: > https://lists.ubuntu.com/archives/kernel-team/2022-September/132966.html > > This led to a off list discussion between myself, Stefan bader, Andrea Righi, > John Johansen, Jay Vosburgh and Tim Gardner: > > Subject: > Please Re-review LSM Stacking Patchset fix - Change Landlock from LSMBLOB_NEEDED > to LSMBLOB_NOT_NEEDED > > The discussion uses largely information found in this SRU template, plus some > additional information from mailing list threads highlighting why it is safe to > move to LSMBLOB_NOT_NEEDED. > > From Matthew Ruffell > https://pastebin.canonical.com/p/hxMgk2tDG9/ > > From John Johansen > https://pastebin.canonical.com/p/ZM4qCMrx2M/ > https://pastebin.canonical.com/p/q6485wDwdt/ > > From Matthew Ruffell > https://pastebin.canonical.com/p/y2HxnDMS6T/ > > From Stefan Bader > https://pastebin.canonical.com/p/CCvVh8SvhB/ > > Now that the misunderstanding has been cleared up, and the patchset has been > double checked and approved by the Security Team, resubmitting the initial > patch as V2. > > Matthew Ruffell (1): > UBUNTU: SAUCE: LSM: Change Landlock from LSMBLOB_NEEDED to > LSMBLOB_NOT_NEEDED > > security/landlock/cred.c | 5 ----- > security/landlock/fs.c | 5 ----- > security/landlock/ptrace.c | 5 ----- > security/landlock/setup.c | 5 +++++ > security/landlock/setup.h | 1 + > 5 files changed, 6 insertions(+), 15 deletions(-) > Applied to jammy:linux/master-next. Thanks. -Stefan
BugLink: https://bugs.launchpad.net/bugs/1987998 [Impact] The Ubuntu kernel carries an out of tree patchet, known as "LSM: Module stacking for AppArmor" upstream, to enable stackable LSMs for containers. The revision the Ubuntu kernel carries is an older one, from 2020, and has some slight divergences from the latest revision in development. One such divergence, is support for Landlock as a stackable LSM. When the stackable LSM patchset was applied, Landlock was still in development and not mainlined yet, and wasn't present in the earlier revision of the "LSM: Module stacking for AppArmor" patchset. Support for this was added by us. There was a minor omission made during enabling support for Landlock. The LSM slot type was marked as LSMBLOB_NEEDED, when it should have been LSMBLOB_NOT_NEEDED. Landlock itself does not provide any of the hooks that use a struct lsmblob, such as secid_to_secctx, secctx_to_secid, inode_getsecid, cred_getsecid, kernel_act_as task_getsecid_subj task_getsecid_obj and ipc_getsecid. When we set .slot = LSMBLOB_NEEDED, this indicates that we need an entry in struct lsmblob, and we need to increment LSMBLOB_ENTRIES by one to fit the entry into the secid array: #define LSMBLOB_ENTRIES ( \ (IS_ENABLED(CONFIG_SECURITY_SELINUX) ? 1 : 0) + \ (IS_ENABLED(CONFIG_SECURITY_SMACK) ? 1 : 0) + \ (IS_ENABLED(CONFIG_SECURITY_APPARMOR) ? 1 : 0) + \ (IS_ENABLED(CONFIG_BPF_LSM) ? 1 : 0)) struct lsmblob { u32 secid[LSMBLOB_ENTRIES]; }; Currently, we don't increment LSMBLOB_ENTRIES by one to make an entry for Landlock, so for the Ubuntu kernel, we can fit a maximum of two entries, one for Apparmor and one for bpf. If you try and configure three LSMs like so and reboot: GRUB_CMDLINE_LINUX_DEFAULT="lsm=landlock,bpf,apparmor" You will receive the following panic: LSM: Security Framework initializing landlock: Up and running. LSM support for eBPF active Kernel panic - not syncing: security_add_hooks Too many LSMs registered. CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.15.0-46-generic #49-Ubuntu Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014 Call Trace: <TASK> show_stack+0x52/0x5c dump_stack_lvl+0x4a/0x63 dump_stack+0x10/0x16 panic+0x149/0x321 security_add_hooks+0x45/0x13a apparmor_init+0x189/0x1ef initialize_lsm+0x54/0x74 ordered_lsm_init+0x379/0x392 security_init+0x40/0x49 start_kernel+0x466/0x4dc x86_64_start_reservations+0x24/0x2a x86_64_start_kernel+0xe4/0xef secondary_startup_64_no_verify+0xc2/0xcb </TASK> ---[ end Kernel panic - not syncing: security_add_hooks Too many LSMs registered. ]--- There is a check added in security_add_hooks() that makes sure that you cannot configure too many LSMs: if (lsmid->slot == LSMBLOB_NEEDED) { if (lsm_slot >= LSMBLOB_ENTRIES) panic("%s Too many LSMs registered.\n", __func__); lsmid->slot = lsm_slot++; init_debug("%s assigned lsmblob slot %d\n", lsmid->lsm, lsmid->slot); } A workaround is to enable no more than 2 LSMs until this is fixed. [Fix] If you read the following mailing list thread on linux-security-modules from May 2021: https://lore.kernel.org/selinux/202105141224.942DE93@keescook/T/ It is explained that Landlock does not provide any of the hooks that use a struct lsmblob, such as secid_to_secctx, secctx_to_secid, inode_getsecid, cred_getsecid, kernel_act_as task_getsecid_subj task_getsecid_obj and ipc_getsecid. I verified this with: ubuntu-jammy$ grep -Rin "secid_to_secctx" security/landlock/ ubuntu-jammy$ grep -Rin "secctx_to_secid" security/landlock/ ubuntu-jammy$ grep -Rin "inode_getsecid" security/landlock/ ubuntu-jammy$ grep -Rin "cred_getsecid" security/landlock/ ubuntu-jammy$ grep -Rin "kernel_act_as" security/landlock/ ubuntu-jammy$ grep -Rin "task_getsecid_subj" security/landlock/ ubuntu-jammy$ grep -Rin "task_getsecid_obj" security/landlock/ ubuntu-jammy$ grep -Rin "ipc_getsecid" security/landlock/ The fix is to change Landlock from LSMBLOB_NEEDED to LSMBLOB_NOT_NEEDED. Due to the "LSM: Module stacking for AppArmor" patchset being 25 patches long, it was impractical to revert just the below patch and reapply with the fix, due to a large amount of conflicts: commit f17b27a2790e72198d2aaf45242453e5a9043049 Author: Casey Schaufler <casey@schaufler-ca.com> Date: Mon Aug 17 16:02:56 2020 -0700 Subject: UBUNTU: SAUCE: LSM: Create and manage the lsmblob data structure. Link: https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/jammy/commit/?id=f17b27a2790e72198d2aaf45242453e5a9043049 So instead, I wrote up a fix that just changes the Landlock LSM slots to follow the latest upstream development, from V37 of the patchset: https://lore.kernel.org/selinux/20220628005611.13106-4-casey@schaufler-ca.com/ I refactored the landlock_lsmid struct to only be in one place, and to be marked as extern from security/landlock/setup.h. [Testcase] Launch a Jammy or Kinetic VM. 1. Edit /etc/default/grub and append the following to GRUB_CMDLINE_LINUX_DEFAULT: "lsm=landlock,bpf,apparmor" 2. sudo update-grub 3. reboot The system will panic on boot. If you install the test kernel from the following ppa: https://launchpad.net/~mruffell/+archive/ubuntu/sf343286-test Instead of a panic occurring, the kernel should display all LSMs initialising, and continue to boot. [ 0.288224] LSM: Security Framework initializing [ 0.289457] landlock: Up and running. [ 0.290290] LSM support for eBPF active [ 0.291189] AppArmor: AppArmor initialized [Where problems could occur] The risk of regression in changing Landlock from LSMBLOB_NEEDED to LSMBLOB_NOT_NEEDED is low, due to Landlock not needing a slot in the secid array in struct lsmblob in the first place. The refactor is minor and unlikely to introduce any issues with Landlock or its security promises. I feel that simply fixing this small bug is less regression risk than reverting the entire 25 patch patchset and applying the latest V37 upstream patchset, which has undergone significant changes from mid 2020. I think its best we consume the newer patchset once it makes its way into mainline in a future kernel instead. If a regression were to occur, users could configure 2 LSMs instead of all 3, or not enable Landlock. [Other Info] This patchset was originally NACKed by Stefan Bader on request of John Johansen due to a minor misunderstanding on how LSMBLOB_NEEDED worked with blob slot registration. ACK: https://lists.ubuntu.com/archives/kernel-team/2022-August/132898.html Applied: https://lists.ubuntu.com/archives/kernel-team/2022-August/132887.html NACK: https://lists.ubuntu.com/archives/kernel-team/2022-September/132965.html Revert: https://lists.ubuntu.com/archives/kernel-team/2022-September/132966.html This led to a off list discussion between myself, Stefan bader, Andrea Righi, John Johansen, Jay Vosburgh and Tim Gardner: Subject: Please Re-review LSM Stacking Patchset fix - Change Landlock from LSMBLOB_NEEDED to LSMBLOB_NOT_NEEDED The discussion uses largely information found in this SRU template, plus some additional information from mailing list threads highlighting why it is safe to move to LSMBLOB_NOT_NEEDED. From Matthew Ruffell https://pastebin.canonical.com/p/hxMgk2tDG9/ From John Johansen https://pastebin.canonical.com/p/ZM4qCMrx2M/ https://pastebin.canonical.com/p/q6485wDwdt/ From Matthew Ruffell https://pastebin.canonical.com/p/y2HxnDMS6T/ From Stefan Bader https://pastebin.canonical.com/p/CCvVh8SvhB/ Now that the misunderstanding has been cleared up, and the patchset has been double checked and approved by the Security Team, resubmitting the initial patch as V2. Matthew Ruffell (1): UBUNTU: SAUCE: LSM: Change Landlock from LSMBLOB_NEEDED to LSMBLOB_NOT_NEEDED security/landlock/cred.c | 5 ----- security/landlock/fs.c | 5 ----- security/landlock/ptrace.c | 5 ----- security/landlock/setup.c | 5 +++++ security/landlock/setup.h | 1 + 5 files changed, 6 insertions(+), 15 deletions(-)