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Message-ID: <8408d8901e9d7ee6b78db4c6cba04b78@ispras.ru>
Date: Tue, 07 Mar 2023 16:01:56 +0300
From: Alexey Izbyshev <izbyshev@...ras.ru>
To: Joey Gouly <joey.gouly@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
Lennart Poettering <lennart@...ttering.net>,
Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>,
Alexander Viro <viro@...iv.linux.org.uk>,
Kees Cook <keescook@...omium.org>,
Szabolcs Nagy <szabolcs.nagy@....com>,
Mark Brown <broonie@...nel.org>,
Jeremy Linton <jeremy.linton@....com>,
Topi Miettinen <toiwoton@...il.com>, linux-mm@...ck.org,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-abi-devel@...ts.sourceforge.net, nd@....com, shuah@...nel.org
Subject: Re: [PATCH v2 1/2] mm: Implement memory-deny-write-execute as a prctl
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@....com>
> Co-developed-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> Cc: Andrew Morton <akpm@...ux-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, ¤t->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, ¤t->mm->flags);
> + else if (test_bit(MMF_HAS_MDWE, ¤t->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, ¤t->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;
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