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Hallyn" , Stephen Smalley , Jeff Xu , =?utf-8?B?VGhpw6liYXVk?= 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" , Vlastimil Babka , Pedro Falcato , David Hildenbrand , linux-mm@kvack.org Subject: Re: [PATCH v2 2/3] proc: query LSMs for introspective mem access (if PROC_MEM_FORCE_ALWAYS) Message-ID: References: <20260825-selinux-pokemem-v2-0-b46bc64916d8@google.com> <20260825-selinux-pokemem-v2-2-b46bc64916d8@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260825-selinux-pokemem-v2-2-b46bc64916d8@google.com> X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 061954000F X-Stat-Signature: bmigmsg5egho3jjx77fkog8875na6ocw X-HE-Tag: 1787749628-990819 X-HE-Meta: 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 D1EQR37y hSCV35C+klksG3wMmaB0c/81++1+h4aGE2n+u+6NYOcYp6nt9LVAiIy/KGDrs271Mbp3lvsxR3TpJKuW95G1st6ANCrPmldWW733kLQ9iLzz3LhqHIGM8TDRauTTmJ5LxJUVmGRl8wSmUXaYFG2/DjeOu1D2lVWlP6v/t/O2oUE9DFHOl883G01qER++kMWeJALX8PkFQldecKoeaw4YFTgS3kiJ2V6g7VkC8vwvo+A93QsLPaKMpKjqIODVWHUQeI4svNwHhX7b9GK7PCS4SeaNrrks9XQVrY3QUKNW5QPi5uoZH0QZs226ZoV5jtJ5mdc7F3Ryt5ADvxSsDIzxhk0Sg+SILEJJ4rpTj2ik0ypbz5VSBYBfSRyZgLtu2KqsBvePabgYt6w2sbJEICUN46h0Eko9l4W57sa6o0+Aey0oMstc= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Aug 25, 2026 at 08:39:18PM +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. > > Signed-off-by: Jann Horn Thanks for the name change! :) Nothing stands out so: Acked-by: Lorenzo Stoakes (ARM) > --- > fs/proc/base.c | 9 +++++++++ > include/linux/lsm_hook_defs.h | 1 + > include/linux/security.h | 6 ++++++ > security/security.c | 20 ++++++++++++++++++++ > 4 files changed, 36 insertions(+) > > diff --git a/fs/proc/base.c b/fs/proc/base.c > index bec6197329dc..dc6fdcb47b79 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; > > @@ -886,6 +893,8 @@ static bool proc_mem_foll_force(struct file *file, struct mm_struct *mm) > } > return ptrace_active; > default: > + if (priv->opened_by_owner) > + return security_mem_foll_force_opened_by_owner(file->f_cred) == 0; > return true; > } > } > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h > index 65c9609ec207..50e3f0abc676 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_opened_by_owner, const struct cred *subject) > 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..74eb876054b0 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_opened_by_owner(const struct cred *subject); > int security_capget(const struct task_struct *target, > kernel_cap_t *effective, > kernel_cap_t *inheritable, > @@ -676,6 +677,11 @@ static inline int security_ptrace_traceme(struct task_struct *parent) > return cap_ptrace_traceme(parent); > } > > +static inline int security_mem_foll_force_opened_by_owner(const struct cred *subject) > +{ > + 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..fff26ff65e07 100644 > --- a/security/security.c > +++ b/security/security.c > @@ -595,6 +595,26 @@ int security_ptrace_traceme(struct task_struct *parent) > return call_int_hook(ptrace_traceme, parent); > } > > +/** > + * security_mem_foll_force_opened_by_owner() - Check if introspective FOLL_FORCE is allowed > + * @subject: credentials of the process accessing its own memory > + * > + * Check if FOLL_FORCE is allowed for accessing process memory through > + * /proc/$pid/mem in the case where the opener's MM was the same as the target > + * MM, meaning the security_ptrace_access_check() hook was bypassed on open(). > + * (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"), this hook still runs.) > + * > + * This is only used when the system is configured with PROC_MEM_FORCE_ALWAYS. > + * > + * Return: Returns 0 if permission is granted. > + */ > +int security_mem_foll_force_opened_by_owner(const struct cred *subject) > +{ > + return call_int_hook(mem_foll_force_opened_by_owner, subject); > +} > + > /** > * security_capget() - Get the capability sets for a process > * @target: target process > > -- > 2.55.0.860.g4b6b3295ed-goog > -- Cheers, Lorenzo