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Hallyn" To: Paul Moore Cc: "Serge Hallyn (AMD)" , Jann Horn , James Morris , Stephen Smalley , Jeff Xu , =?iso-8859-1?Q?Thi=E9baud?= Weksteen , Alexander Viro , Christian Brauner , Jan Kara , linux-fsdevel@vger.kernel.org, linux-security-module@vger.kernel.org, Ondrej Mosnacek , selinux@vger.kernel.org, Andrew Morton , "Liam R. Howlett" , Lorenzo Stoakes , Vlastimil Babka , Pedro Falcato , David Hildenbrand , linux-mm@kvack.org Subject: Re: [PATCH v3 2/3] proc: query LSMs for introspective mem access (if PROC_MEM_FORCE_ALWAYS) Message-ID: References: <20260907-selinux-pokemem-v3-0-0bafbaeafe50@google.com> <20260907-selinux-pokemem-v3-2-0bafbaeafe50@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 71CEA180002 X-Stat-Signature: qzc63w9unp1e6yq5ctcdfoy1ziyuhah7 X-HE-Tag: 1789494868-281693 X-HE-Meta: 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 J3I2xBW5 LslZmjfp47voUBQ8BaQjoWSmAbNFbhC/Xc4jqpxqGVMQimVVHgM0xpSdZ6Avcs8AwzeiU43aaZtulhAxv96qJr4HdWabavPI1CM0mJ5Gw4r51GdQFkZ36mtUPUECQ4tMRkLXyNI82A20Xbz64L+NtOG1Ob/rfrya2fU3EsH1NQQERQsGJypG8rN/veWHJwA++dqvIsoOgqcRLMN9kZrM1WzK6IRZbF0eabi9TnERqQzCwkEv6SPmWl/r3MfzMrMMfCcntrP72vZZNVJ9WLmErGkhCOE5OAL3utrZAnTIcaQBe9x+4uGLTAnPQIFR47DS2rlnPVnJnfO9wNzWD0Nvh6Eeqt+cZcJoIL4/WQXB45DeVAfDGQQIjKaYBsOfoMAGLVKOr Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Sep 15, 2026 at 01:24:27PM -0400, Paul Moore wrote: > On Tue, Sep 15, 2026 at 1:04 PM Serge Hallyn (AMD) wrote: > > On Mon, Sep 07, 2026 at 11:00:17PM +0200, Jann Horn wrote: > > > If the system is running with PROC_MEM_FORCE_ALWAYS, LSMs currently have no > > > good opportunity to block a process from overwriting read-only code in its > > > own address space through FOLL_FORCE writes via /proc/self/mem. > > > The security_ptrace_access_check() LSM hook is bypassed when a process > > > opens /proc/self/mem because this is considered "introspection". > > > > > > This causes a hole in SELinux EXECMEM enforcement, which tries to ensure > > > that a process cannot create executable anonymous pages. > > > > > > PROC_MEM_FORCE_PTRACE prevents that and ensures that such FOLL_FORCE > > > accesses are only possible when the LSM allows ptrace() attachment; but it > > > is unclear how quickly PROC_MEM_FORCE_PTRACE can be deployed in > > > environments running lots of third-party code, such as Android. > > > > > > So, introduce a new LSM hook that can forbid FOLL_FORCE specifically for > > > such "introspective" accesses. > > > > > > Acked-by: Lorenzo Stoakes (ARM) > > > Acked-by: David Hildenbrand (Arm) > > > Signed-off-by: Jann Horn > > > --- > > > fs/proc/base.c | 14 ++++++++++++-- > > > include/linux/lsm_hook_defs.h | 1 + > > > include/linux/security.h | 7 +++++++ > > > security/security.c | 25 +++++++++++++++++++++++++ > > > 4 files changed, 45 insertions(+), 2 deletions(-) > > > > > > diff --git a/fs/proc/base.c b/fs/proc/base.c > > > index bec6197329dc..295b21203c7f 100644 > > > --- a/fs/proc/base.c > > > +++ b/fs/proc/base.c > > > @@ -851,6 +851,11 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode) > > > /* private_data for proc_mem_operations */ > > > struct mem_private { > > > struct mm_struct *mm; > > > + /* > > > + * Was the ptrace access check on open bypassed because the opener used > > > + * the same MM (introspection)? > > > + */ > > > + bool opened_by_owner; > > > }; > > > > > > static int mem_open(struct inode *inode, struct file *file) > > > @@ -864,12 +869,14 @@ static int mem_open(struct inode *inode, struct file *file) > > > priv->mm = proc_mem_open(inode, PTRACE_MODE_ATTACH); > > > if (IS_ERR_OR_NULL(priv->mm)) > > > return priv->mm ? PTR_ERR(priv->mm) : -ESRCH; > > > + priv->opened_by_owner = priv->mm == current->mm; > > > file->private_data = no_free_ptr(priv); > > > return 0; > > > } > > > > > > static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm) > > > { > > > + struct mem_private *priv = file->private_data; > > > struct task_struct *task; > > > bool ptrace_active = false; > > > > > > @@ -884,10 +891,13 @@ static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm) > > > READ_ONCE(task->parent) == current; > > > put_task_struct(task); > > > } > > > - return ptrace_active; > > > + if (!ptrace_active) > > > + return false; > > > + break; > > > default: > > > - return true; > > > + break; > > > } > > > + return security_mem_foll_force(file->f_cred, priv->opened_by_owner) == 0; > > > } > > > > > > static ssize_t mem_rw(struct file *file, char __user *buf, > > > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h > > > index 65c9609ec207..12f84a1e6fab 100644 > > > --- a/include/linux/lsm_hook_defs.h > > > +++ b/include/linux/lsm_hook_defs.h > > > @@ -36,6 +36,7 @@ LSM_HOOK(int, 0, binder_transfer_file, const struct cred *from, > > > LSM_HOOK(int, 0, ptrace_access_check, struct task_struct *child, > > > unsigned int mode) > > > LSM_HOOK(int, 0, ptrace_traceme, struct task_struct *parent) > > > +LSM_HOOK(int, 0, mem_foll_force, const struct cred *subject, bool opened_by_owner) > > > LSM_HOOK(int, 0, capget, const struct task_struct *target, kernel_cap_t *effective, > > > kernel_cap_t *inheritable, kernel_cap_t *permitted) > > > LSM_HOOK(int, 0, capset, struct cred *new, const struct cred *old, > > > diff --git a/include/linux/security.h b/include/linux/security.h > > > index 153e9043058f..e8bc2e644241 100644 > > > --- a/include/linux/security.h > > > +++ b/include/linux/security.h > > > @@ -338,6 +338,7 @@ int security_binder_transfer_file(const struct cred *from, > > > const struct cred *to, const struct file *file); > > > int security_ptrace_access_check(struct task_struct *child, unsigned int mode); > > > int security_ptrace_traceme(struct task_struct *parent); > > > +int security_mem_foll_force(const struct cred *subject, bool opened_by_owner); > > > int security_capget(const struct task_struct *target, > > > kernel_cap_t *effective, > > > kernel_cap_t *inheritable, > > > @@ -676,6 +677,12 @@ static inline int security_ptrace_traceme(struct task_struct *parent) > > > return cap_ptrace_traceme(parent); > > > } > > > > > > +static inline int security_mem_foll_force(const struct cred *subject, > > > + bool opened_by_owner) > > > +{ > > > + return 0; > > > +} > > > + > > > static inline int security_capget(const struct task_struct *target, > > > kernel_cap_t *effective, > > > kernel_cap_t *inheritable, > > > diff --git a/security/security.c b/security/security.c > > > index 71aea8fdf014..2cde1efdb7a6 100644 > > > --- a/security/security.c > > > +++ b/security/security.c > > > @@ -595,6 +595,31 @@ int security_ptrace_traceme(struct task_struct *parent) > > > return call_int_hook(ptrace_traceme, parent); > > > } > > > > > > +/** > > > + * security_mem_foll_force() - Check if FOLL_FORCE is allowed > > > + * @subject: credentials using which /proc/$pid/mem was opened > > > + * @opened_by_owner: whether checks on open() were bypassed because the opener > > > + * has the same MM as the target > > > + * > > > + * Check if FOLL_FORCE is allowed for accessing process memory through > > > + * /proc/$pid/mem. opened_by_owner signals whether the opener's MM was the same > > > + * as the target MM, meaning the security_ptrace_access_check() hook was > > > + * bypassed on open(). > > > + * (Current current->mm does not matter for this; for example, if write() is > > > + * called on an FD that was received from another process which obtained it with > > > + * open("/proc/self/mem"), @opened_by_owner is still true.) > > > + * > > > + * Note that this hook is only designed to be useful in the opened_by_owner > > > + * case, where the subject credentials effectively also describe the object. > > > > Given this, would it make more sense to call the hook something > > like `security_mem_foll_force_self()` and only call it in the > > opened_by_owner==true case? > > Sometimes we can't avoid it, but in general I'd like to see us avoid > calling LSM hooks conditionally, I'd much prefer the hook > implementation (either at the LSM framework level or the individual > LSM) do the conditional check. See my comments on the previous > revision. Ah. Sorry, I had missed that. +1.