| Message ID | 20210310161000.382796-1-mic@digikod.net (mailing list archive) |
|---|---|
| Headers | show |
| Series | Unprivileged chroot | expand |
On 3/10/2021 8:09 AM, Mickaël Salaün wrote: > Hi, > > The chroot system call is currently limited to be used by processes with > the CAP_SYS_CHROOT capability. This protects against malicious > procesess willing to trick SUID-like binaries. The following patch > allows unprivileged users to safely use chroot(2). Mount namespaces have pretty well obsoleted chroot(). CAP_SYS_CHROOT is one of the few fine grained capabilities. We're still finding edge cases (e.g. ptrace) where no_new_privs is imperfect. I doesn't seem that there is a compelling reason to remove the privilege requirement on chroot(). > > This patch is a follow-up of a previous one sent by Andy Lutomirski some > time ago: > https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.luto@amacapital.net/ > > This patch can be applied on top of v5.12-rc2 . I would really > appreciate constructive reviews. > > Regards, > > Mickaël Salaün (1): > fs: Allow no_new_privs tasks to call chroot(2) > > fs/open.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++--- > 1 file changed, 61 insertions(+), 3 deletions(-) > > > base-commit: a38fd8748464831584a19438cbb3082b5a2dab15
On 10/03/2021 18:22, Casey Schaufler wrote: > On 3/10/2021 8:09 AM, Mickaël Salaün wrote: >> Hi, >> >> The chroot system call is currently limited to be used by processes with >> the CAP_SYS_CHROOT capability. This protects against malicious >> procesess willing to trick SUID-like binaries. The following patch >> allows unprivileged users to safely use chroot(2). > > Mount namespaces have pretty well obsoleted chroot(). CAP_SYS_CHROOT is > one of the few fine grained capabilities. We're still finding edge cases > (e.g. ptrace) where no_new_privs is imperfect. I doesn't seem that there > is a compelling reason to remove the privilege requirement on chroot(). What is the link between chroot and ptrace? What is interesting with CAP_SYS_CHROOT? > >> >> This patch is a follow-up of a previous one sent by Andy Lutomirski some >> time ago: >> https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.luto@amacapital.net/ >> >> This patch can be applied on top of v5.12-rc2 . I would really >> appreciate constructive reviews. >> >> Regards, >> >> Mickaël Salaün (1): >> fs: Allow no_new_privs tasks to call chroot(2) >> >> fs/open.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++--- >> 1 file changed, 61 insertions(+), 3 deletions(-) >> >> >> base-commit: a38fd8748464831584a19438cbb3082b5a2dab15 >
On 3/10/2021 10:17 AM, Mickaël Salaün wrote: > On 10/03/2021 18:22, Casey Schaufler wrote: >> On 3/10/2021 8:09 AM, Mickaël Salaün wrote: >>> Hi, >>> >>> The chroot system call is currently limited to be used by processes with >>> the CAP_SYS_CHROOT capability. This protects against malicious >>> procesess willing to trick SUID-like binaries. The following patch >>> allows unprivileged users to safely use chroot(2). >> Mount namespaces have pretty well obsoleted chroot(). CAP_SYS_CHROOT is >> one of the few fine grained capabilities. We're still finding edge cases >> (e.g. ptrace) where no_new_privs is imperfect. I doesn't seem that there >> is a compelling reason to remove the privilege requirement on chroot(). > What is the link between chroot and ptrace? The possibility of sophisticated interactions with no_new_privs. > What is interesting with CAP_SYS_CHROOT? CAP_SYS_CHROOT is specific to chroot. It doesn't give you privilege beyond what you expect, unlike CAP_CHOWN or CAP_SYS_ADMIN. Making chroot unprivileged is silly when it's possibly the best example of how the capability mechanism is supposed to work. > >>> This patch is a follow-up of a previous one sent by Andy Lutomirski some >>> time ago: >>> https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.luto@amacapital.net/ >>> >>> This patch can be applied on top of v5.12-rc2 . I would really >>> appreciate constructive reviews. >>> >>> Regards, >>> >>> Mickaël Salaün (1): >>> fs: Allow no_new_privs tasks to call chroot(2) >>> >>> fs/open.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++--- >>> 1 file changed, 61 insertions(+), 3 deletions(-) >>> >>> >>> base-commit: a38fd8748464831584a19438cbb3082b5a2dab15
On 10/03/2021 21:59, Casey Schaufler wrote: > On 3/10/2021 10:17 AM, Mickaël Salaün wrote: >> On 10/03/2021 18:22, Casey Schaufler wrote: >>> On 3/10/2021 8:09 AM, Mickaël Salaün wrote: >>>> Hi, >>>> >>>> The chroot system call is currently limited to be used by processes with >>>> the CAP_SYS_CHROOT capability. This protects against malicious >>>> procesess willing to trick SUID-like binaries. The following patch >>>> allows unprivileged users to safely use chroot(2). >>> Mount namespaces have pretty well obsoleted chroot(). CAP_SYS_CHROOT is >>> one of the few fine grained capabilities. We're still finding edge cases >>> (e.g. ptrace) where no_new_privs is imperfect. I doesn't seem that there >>> is a compelling reason to remove the privilege requirement on chroot(). >> What is the link between chroot and ptrace? > > The possibility of sophisticated interactions with no_new_privs. Would you mind giving some practical examples? > >> What is interesting with CAP_SYS_CHROOT? > > CAP_SYS_CHROOT is specific to chroot. It doesn't give you privilege > beyond what you expect, unlike CAP_CHOWN or CAP_SYS_ADMIN. Making chroot > unprivileged is silly when it's possibly the best example of how the > capability mechanism is supposed to work. Why would it be silly to make the use of this feature safe for any processes instead of giving the right (with CAP_SYS_CHROOT) to some processes to use it unsafely? > >> >>>> This patch is a follow-up of a previous one sent by Andy Lutomirski some >>>> time ago: >>>> https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.luto@amacapital.net/ >>>> >>>> This patch can be applied on top of v5.12-rc2 . I would really >>>> appreciate constructive reviews. >>>> >>>> Regards, >>>> >>>> Mickaël Salaün (1): >>>> fs: Allow no_new_privs tasks to call chroot(2) >>>> >>>> fs/open.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++--- >>>> 1 file changed, 61 insertions(+), 3 deletions(-) >>>> >>>> >>>> base-commit: a38fd8748464831584a19438cbb3082b5a2dab15 >