[PATCH v2 1/2] mm: Implement memory-deny-write-execute as a prctl

Tue Mar 7 05:01:56 PST 2023

On 2023-01-19 19:03, Joey Gouly wrote:
> The aim of such policy is to prevent a user task from creating an
> executable mapping that is also writeable.
> 
> An example of mmap() returning -EACCESS if the policy is enabled:
> 
> 	mmap(0, size, PROT_READ | PROT_WRITE | PROT_EXEC, flags, 0, 0);
> 
> Similarly, mprotect() would return -EACCESS below:
> 
> 	addr = mmap(0, size, PROT_READ | PROT_EXEC, flags, 0, 0);
> 	mprotect(addr, size, PROT_READ | PROT_WRITE | PROT_EXEC);
> 
> The BPF filter that systemd MDWE uses is stateless, and disallows
> mprotect() with PROT_EXEC completely. This new prctl allows PROT_EXEC 
> to
> be enabled if it was already PROT_EXEC, which allows the following 
> case:
> 
> 	addr = mmap(0, size, PROT_READ | PROT_EXEC, flags, 0, 0);
> 	mprotect(addr, size, PROT_READ | PROT_EXEC | PROT_BTI);
> 
> where PROT_BTI enables branch tracking identification on arm64.
> 
> Signed-off-by: Joey Gouly <joey.gouly at arm.com>
> Co-developed-by: Catalin Marinas <catalin.marinas at arm.com>
> Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> ---
>  include/linux/mman.h           | 34 ++++++++++++++++++++++++++++++++++
>  include/linux/sched/coredump.h |  6 +++++-
>  include/uapi/linux/prctl.h     |  6 ++++++
>  kernel/sys.c                   | 33 +++++++++++++++++++++++++++++++++
>  mm/mmap.c                      | 10 ++++++++++
>  mm/mprotect.c                  |  5 +++++
>  6 files changed, 93 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mman.h b/include/linux/mman.h
> index 58b3abd457a3..cee1e4b566d8 100644
> --- a/include/linux/mman.h
> +++ b/include/linux/mman.h
> @@ -156,4 +156,38 @@ calc_vm_flag_bits(unsigned long flags)
>  }
> 
>  unsigned long vm_commit_limit(void);
> +
> +/*
> + * Denies creating a writable executable mapping or gaining
> executable permissions.
> + *
> + * This denies the following:
> + *
> + * 	a)	mmap(PROT_WRITE | PROT_EXEC)
> + *
> + *	b)	mmap(PROT_WRITE)
> + *		mprotect(PROT_EXEC)
> + *
> + *	c)	mmap(PROT_WRITE)
> + *		mprotect(PROT_READ)
> + *		mprotect(PROT_EXEC)
> + *
> + * But allows the following:
> + *
> + *	d)	mmap(PROT_READ | PROT_EXEC)
> + *		mmap(PROT_READ | PROT_EXEC | PROT_BTI)
> + */
> +static inline bool map_deny_write_exec(struct vm_area_struct *vma,
> unsigned long vm_flags)
> +{
> +	if (!test_bit(MMF_HAS_MDWE, &current->mm->flags))
> +		return false;
> +
> +	if ((vm_flags & VM_EXEC) && (vm_flags & VM_WRITE))
> +		return true;
> +
> +	if (!(vma->vm_flags & VM_EXEC) && (vm_flags & VM_EXEC))
> +		return true;
> +
> +	return false;
> +}
> +
>  #endif /* _LINUX_MMAN_H */
> diff --git a/include/linux/sched/coredump.h 
> b/include/linux/sched/coredump.h
> index 8270ad7ae14c..0e17ae7fbfd3 100644
> --- a/include/linux/sched/coredump.h
> +++ b/include/linux/sched/coredump.h
> @@ -81,9 +81,13 @@ static inline int get_dumpable(struct mm_struct *mm)
>   * lifecycle of this mm, just for simplicity.
>   */
>  #define MMF_HAS_PINNED		27	/* FOLL_PIN has run, never cleared */
> +
> +#define MMF_HAS_MDWE		28
> +#define MMF_HAS_MDWE_MASK	(1 << MMF_HAS_MDWE)
> +
>  #define MMF_DISABLE_THP_MASK	(1 << MMF_DISABLE_THP)
> 
>  #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
> -				 MMF_DISABLE_THP_MASK)
> +				 MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK)
> 
>  #endif /* _LINUX_SCHED_COREDUMP_H */
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index a5e06dcbba13..1312a137f7fb 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -281,6 +281,12 @@ struct prctl_mm_map {
>  # define PR_SME_VL_LEN_MASK		0xffff
>  # define PR_SME_VL_INHERIT		(1 << 17) /* inherit across exec */
> 
> +/* Memory deny write / execute */
> +#define PR_SET_MDWE			65
> +# define PR_MDWE_REFUSE_EXEC_GAIN	1
> +
> +#define PR_GET_MDWE			66
> +
>  #define PR_SET_VMA		0x53564d41
>  # define PR_SET_VMA_ANON_NAME		0
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 5fd54bf0e886..b3cab94545ed 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2348,6 +2348,33 @@ static int prctl_set_vma(unsigned long opt,
> unsigned long start,
>  }
>  #endif /* CONFIG_ANON_VMA_NAME */
> 
> +static inline int prctl_set_mdwe(unsigned long bits, unsigned long 
> arg3,
> +				 unsigned long arg4, unsigned long arg5)
> +{
> +	if (arg3 || arg4 || arg5)
> +		return -EINVAL;
> +
> +	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN))
> +		return -EINVAL;
> +
> +	if (bits & PR_MDWE_REFUSE_EXEC_GAIN)
> +		set_bit(MMF_HAS_MDWE, &current->mm->flags);
> +	else if (test_bit(MMF_HAS_MDWE, &current->mm->flags))
> +		return -EPERM; /* Cannot unset the flag */
> +
> +	return 0;
> +}
> +
> +static inline int prctl_get_mdwe(unsigned long arg2, unsigned long 
> arg3,
> +				 unsigned long arg4, unsigned long arg5)
> +{
> +	if (arg2 || arg3 || arg4 || arg5)
> +		return -EINVAL;
> +
> +	return test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
> +		PR_MDWE_REFUSE_EXEC_GAIN : 0;
> +}
> +
>  SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned 
> long, arg3,
>  		unsigned long, arg4, unsigned long, arg5)
>  {
> @@ -2623,6 +2650,12 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned
> long, arg2, unsigned long, arg3,
>  		error = sched_core_share_pid(arg2, arg3, arg4, arg5);
>  		break;
>  #endif
> +	case PR_SET_MDWE:
> +		error = prctl_set_mdwe(arg2, arg3, arg4, arg5);
> +		break;
> +	case PR_GET_MDWE:
> +		error = prctl_get_mdwe(arg2, arg3, arg4, arg5);
> +		break;
>  	case PR_SET_VMA:
>  		error = prctl_set_vma(arg2, arg3, arg4, arg5);
>  		break;
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 87d929316d57..99a4d9e2b0d8 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2665,6 +2665,16 @@ unsigned long mmap_region(struct file *file,
> unsigned long addr,
>  		vma_set_anonymous(vma);
>  	}
> 
> +	if (map_deny_write_exec(vma, vma->vm_flags)) {
> +		error = -EACCES;
> +		if (file)
> +			goto close_and_free_vma;
> +		else if (vma->vm_file)
> +			goto unmap_and_free_vma;
> +		else
> +			goto free_vma;
> +	}
> +

Why is the cleanup dispatch logic duplicated here, instead of simply 
doing "goto close_and_free_vma" (where basically the same dispatch is 
done)?

>  	/* Allow architectures to sanity-check the vm_flags */
>  	if (!arch_validate_flags(vma->vm_flags)) {
>  		error = -EINVAL;
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 908df12caa26..bc0587df042f 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -762,6 +762,11 @@ static int do_mprotect_pkey(unsigned long start,
> size_t len,
>  			break;
>  		}
> 
> +		if (map_deny_write_exec(vma, newflags)) {
> +			error = -EACCES;
> +			goto out;
> +		}
> +

Why does this check use "goto out", thereby skipping post-loop cleanup, 
instead of "break" like all other checks? This looks like a bug to me.

Alexey
>  		/* Allow architectures to sanity-check the new flags */
>  		if (!arch_validate_flags(newflags)) {
>  			error = -EINVAL;