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[198.0.35.241]) by smtp.gmail.com with ESMTPSA id a3-20020aa78643000000b0068874e1faf2sm2069987pfo.56.2023.08.25.15.38.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 25 Aug 2023 15:38:36 -0700 (PDT) Date: Fri, 25 Aug 2023 15:38:36 -0700 From: Kees Cook To: Florent Revest Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, catalin.marinas@arm.com, anshuman.khandual@arm.com, joey.gouly@arm.com, mhocko@suse.com, david@redhat.com, peterx@redhat.com, izbyshev@ispras.ru, broonie@kernel.org, szabolcs.nagy@arm.com, kpsingh@kernel.org, gthelen@google.com, toiwoton@gmail.com Subject: Re: [PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl Message-ID: <202308251535.551E797B1@keescook> References: <20230704153630.1591122-1-revest@chromium.org> <20230704153630.1591122-5-revest@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230704153630.1591122-5-revest@chromium.org> X-Stat-Signature: f46xq15ny3aipoqmb7enptj6anw3panz X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 357DAC0018 X-Rspam-User: X-HE-Tag: 1693003117-693305 X-HE-Meta: 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 3y1fLuxr oc5luTmL2VTvUJAo36F41xhRtsHdm4eDsVEF3LPWzPqyouJ7UWUZBZxs6pSIXcPbb3Y2xqcuHGE/fG59FXaDgGFN/AuMZcKOVHP5HEwt4sl87mEEvhHMkt4bJJ2DOhkQuDN+uGrjfnoah+8O6Pe9D0/djGClbXR/qNhTX+Q2DagENsHbP4OUwPQFrSUkPziZGSh3QaHUKGNwPPYBAk3MWdm8aO3y4CFqolvhM2a4pkhbiMG7k88tTQASFUAV68VT/09D1wqYL86vK9WF2OfIiNKOyNrkpmNW7DsiwaGqgti5PeppLvHAA9SEFmF2QAWznlZ62C+UaW+C04S7mUF5i0PzFDf2WxrnR5F18UJT4avNo8SETUbUv5YZKbfnBVIo7DSVsUEBeg+ASR4dp8oHm6z7tEb8dSLzQOS6YAPJNXIfidAzndAAJwWhZoVEYBgOVuOrwK6x8lFGRehI= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Jul 04, 2023 at 05:36:28PM +0200, Florent Revest wrote: > This extends the current PR_SET_MDWE prctl arg with a bit to indicate > that the process doesn't want MDWE protection to propagate to children. > > To implement this no-inherit mode, the tag in current->mm->flags must be > absent from MMF_INIT_MASK. This means that the encoding for "MDWE but > without inherit" is different in the prctl than in the mm flags. This > leads to a bit of bit-mangling in the prctl implementation. > > Signed-off-by: Florent Revest > --- > include/linux/sched/coredump.h | 10 ++++++++++ > include/uapi/linux/prctl.h | 1 + > kernel/fork.c | 2 +- > kernel/sys.c | 32 ++++++++++++++++++++++++++------ > tools/include/uapi/linux/prctl.h | 1 + > 5 files changed, 39 insertions(+), 7 deletions(-) > > diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h > index 0ee96ea7a0e9..1b37fa8fc723 100644 > --- a/include/linux/sched/coredump.h > +++ b/include/linux/sched/coredump.h > @@ -91,4 +91,14 @@ static inline int get_dumpable(struct mm_struct *mm) > MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK) > > #define MMF_VM_MERGE_ANY 29 > +#define MMF_HAS_MDWE_NO_INHERIT 30 > + > +static inline unsigned long mmf_init_flags(unsigned long flags) > +{ > + if (flags & (1UL << MMF_HAS_MDWE_NO_INHERIT)) > + flags &= ~((1UL << MMF_HAS_MDWE) | > + (1UL << MMF_HAS_MDWE_NO_INHERIT)); > + return flags & MMF_INIT_MASK; > +} > + > #endif /* _LINUX_SCHED_COREDUMP_H */ > diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h > index 6e9af6cbc950..dacbe824e7c3 100644 > --- a/include/uapi/linux/prctl.h > +++ b/include/uapi/linux/prctl.h > @@ -284,6 +284,7 @@ struct prctl_mm_map { > /* Memory deny write / execute */ > #define PR_SET_MDWE 65 > # define PR_MDWE_REFUSE_EXEC_GAIN (1UL << 0) > +# define PR_MDWE_NO_INHERIT (1UL << 1) > > #define PR_GET_MDWE 66 > > diff --git a/kernel/fork.c b/kernel/fork.c > index d17995934eb4..bc3c762d378f 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -1284,7 +1284,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p, > hugetlb_count_init(mm); > > if (current->mm) { > - mm->flags = current->mm->flags & MMF_INIT_MASK; > + mm->flags = mmf_init_flags(current->mm->flags); > mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK; > } else { > mm->flags = default_dump_filter; > diff --git a/kernel/sys.c b/kernel/sys.c > index 339fee3eff6a..1a2dc3da43ea 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -2362,19 +2362,41 @@ static int prctl_set_vma(unsigned long opt, unsigned long start, > } > #endif /* CONFIG_ANON_VMA_NAME */ > > +static inline unsigned long get_current_mdwe(void) > +{ > + unsigned long ret = 0; > + > + if (test_bit(MMF_HAS_MDWE, ¤t->mm->flags)) > + ret |= PR_MDWE_REFUSE_EXEC_GAIN; > + if (test_bit(MMF_HAS_MDWE_NO_INHERIT, ¤t->mm->flags)) > + ret |= PR_MDWE_NO_INHERIT; > + > + return ret; > +} > + > static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3, > unsigned long arg4, unsigned long arg5) > { > + unsigned long current_bits; > + > if (arg3 || arg4 || arg5) > return -EINVAL; > > - if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN)) > + if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT)) > + return -EINVAL; > + > + /* NO_INHERIT only makes sense with REFUSE_EXEC_GAIN */ > + if (bits & PR_MDWE_NO_INHERIT && !(bits & PR_MDWE_REFUSE_EXEC_GAIN)) > return -EINVAL; > > + current_bits = get_current_mdwe(); > + if (current_bits && current_bits != bits) > + return -EPERM; /* Cannot unset the flags */ I was pondering why PR_MDWE_NO_INHERIT can't be unset, but I guess it doesn't matter. Anything forked with have it off, and any process wanting to launch stuff before locking down can just skip running the prctl() until later. > + > + if (bits & PR_MDWE_NO_INHERIT) > + set_bit(MMF_HAS_MDWE_NO_INHERIT, ¤t->mm->flags); > if (bits & PR_MDWE_REFUSE_EXEC_GAIN) > set_bit(MMF_HAS_MDWE, ¤t->mm->flags); > - else if (test_bit(MMF_HAS_MDWE, ¤t->mm->flags)) > - return -EPERM; /* Cannot unset the flag */ > > return 0; > } > @@ -2384,9 +2406,7 @@ static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3, > { > if (arg2 || arg3 || arg4 || arg5) > return -EINVAL; > - > - return test_bit(MMF_HAS_MDWE, ¤t->mm->flags) ? > - PR_MDWE_REFUSE_EXEC_GAIN : 0; > + return (int)get_current_mdwe(); > } > > static int prctl_get_auxv(void __user *addr, unsigned long len) > diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h > index 6e9af6cbc950..dacbe824e7c3 100644 > --- a/tools/include/uapi/linux/prctl.h > +++ b/tools/include/uapi/linux/prctl.h > @@ -284,6 +284,7 @@ struct prctl_mm_map { > /* Memory deny write / execute */ > #define PR_SET_MDWE 65 > # define PR_MDWE_REFUSE_EXEC_GAIN (1UL << 0) > +# define PR_MDWE_NO_INHERIT (1UL << 1) > > #define PR_GET_MDWE 66 > > -- > 2.41.0.255.g8b1d071c50-goog > Reviewed-by: Kees Cook -- Kees Cook