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Date: Mon, 11 May 2020 14:05:36 +0900
From: Masami Hiramatsu <mhiramat@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: x86@...nel.org, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-parisc@...r.kernel.org, linux-um@...ts.infradead.org,
netdev@...r.kernel.org, bpf@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 12/15] maccess: always use strict semantics for
probe_kernel_read
Hi Christoph,
At first, thank you for your work on cleaning up these functions!
On Wed, 6 May 2020 08:22:20 +0200
Christoph Hellwig <hch@....de> wrote:
> Except for historical confusion in the kprobes/uprobes and bpf tracers
> there is no good reason to ever allow user memory accesses from
> probe_kernel_read.
Yes, thus now trace_kprobe supports "ustring" type for accessing
user space memory. (If the address spaces are overwrapped, we have
no way to distinguish whether an address is kernel or user)
> Make the tracers fall back to a probe_user_read
> if the probe_kernel_read falls to keep the core API clean.
For trace_kprobes doesn't need to fall back. User must specify
the probe should be read from user space or kernel space. This is
because it has fetch_store_string_user() and probe_mem_read_user()
variants.
Thank you,
>
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
> arch/parisc/lib/memcpy.c | 3 +--
> arch/um/kernel/maccess.c | 3 +--
> arch/x86/mm/maccess.c | 5 +----
> include/linux/uaccess.h | 4 +---
> kernel/trace/bpf_trace.c | 20 +++++++++++++------
> kernel/trace/trace_kprobe.c | 11 ++++++++++-
> mm/maccess.c | 39 ++++++-------------------------------
> 7 files changed, 34 insertions(+), 51 deletions(-)
>
> diff --git a/arch/parisc/lib/memcpy.c b/arch/parisc/lib/memcpy.c
> index 5ef648bd33119..9fe662b3b5604 100644
> --- a/arch/parisc/lib/memcpy.c
> +++ b/arch/parisc/lib/memcpy.c
> @@ -57,8 +57,7 @@ void * memcpy(void * dst,const void *src, size_t count)
> EXPORT_SYMBOL(raw_copy_in_user);
> EXPORT_SYMBOL(memcpy);
>
> -bool probe_kernel_read_allowed(void *dst, const void *unsafe_src, size_t size,
> - bool strict)
> +bool probe_kernel_read_allowed(void *dst, const void *unsafe_src, size_t size)
> {
> if ((unsigned long)unsafe_src < PAGE_SIZE)
> return false;
> diff --git a/arch/um/kernel/maccess.c b/arch/um/kernel/maccess.c
> index 90a1bec923158..734f3d7e57c0f 100644
> --- a/arch/um/kernel/maccess.c
> +++ b/arch/um/kernel/maccess.c
> @@ -7,8 +7,7 @@
> #include <linux/kernel.h>
> #include <os.h>
>
> -bool probe_kernel_read_allowed(void *dst, const void *src, size_t size,
> - bool strict)
> +bool probe_kernel_read_allowed(void *dst, const void *src, size_t size)
> {
> void *psrc = (void *)rounddown((unsigned long)src, PAGE_SIZE);
>
> diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
> index 5c323ab187b27..a1bd81677aa72 100644
> --- a/arch/x86/mm/maccess.c
> +++ b/arch/x86/mm/maccess.c
> @@ -26,10 +26,7 @@ static __always_inline bool invalid_probe_range(u64 vaddr)
> }
> #endif
>
> -bool probe_kernel_read_allowed(void *dst, const void *unsafe_src, size_t size,
> - bool strict)
> +bool probe_kernel_read_allowed(void *dst, const void *unsafe_src, size_t size)
> {
> - if (!strict)
> - return true;
> return !invalid_probe_range((unsigned long)unsafe_src);
> }
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 09d6e358883cc..99e2c2a41164a 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -301,11 +301,9 @@ copy_struct_from_user(void *dst, size_t ksize, const void __user *src,
> return 0;
> }
>
> -bool probe_kernel_read_allowed(void *dst, const void *unsafe_src,
> - size_t size, bool strict);
> +bool probe_kernel_read_allowed(void *dst, const void *unsafe_src, size_t size);
>
> extern long probe_kernel_read(void *dst, const void *src, size_t size);
> -extern long probe_kernel_read_strict(void *dst, const void *src, size_t size);
> extern long probe_user_read(void *dst, const void __user *src, size_t size);
>
> extern long notrace probe_kernel_write(void *dst, const void *src, size_t size);
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index ffe841433caa1..f694befe8ec9b 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -183,12 +183,20 @@ bpf_probe_read_kernel_common(void *dst, u32 size, const void *unsafe_ptr,
> int ret = security_locked_down(LOCKDOWN_BPF_READ);
>
> if (unlikely(ret < 0))
> - goto out;
> - ret = compat ? probe_kernel_read(dst, unsafe_ptr, size) :
> - probe_kernel_read_strict(dst, unsafe_ptr, size);
> - if (unlikely(ret < 0))
> -out:
> - memset(dst, 0, size);
> + goto fail;
> +
> + ret = probe_kernel_read(dst, unsafe_ptr, size);
> + if (unlikely(ret < 0)) {
> + if (compat)
> + ret = probe_user_read(dst,
> + (__force const void __user *)unsafe_ptr, size);
> + if (unlikely(ret < 0))
> + goto fail;
> + }
> +
> + return 0;
> +fail:
> + memset(dst, 0, size);
> return ret;
> }
>
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 525d12137325c..1300c9fd5c755 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -1203,6 +1203,9 @@ fetch_store_strlen(unsigned long addr)
>
> do {
> ret = probe_kernel_read(&c, (u8 *)addr + len, 1);
> + if (ret)
> + ret = probe_user_read(&c,
> + (__force u8 __user *)addr + len, 1);
> len++;
> } while (c && ret == 0 && len < MAX_STRING_SIZE);
>
> @@ -1275,7 +1278,13 @@ fetch_store_string_user(unsigned long addr, void *dest, void *base)
> static nokprobe_inline int
> probe_mem_read(void *dest, void *src, size_t size)
> {
> - return probe_kernel_read(dest, src, size);
> + int ret;
> +
> + ret = probe_kernel_read(dest, src, size);
> + if (ret)
> + ret = probe_user_read(dest, (__force const void __user *)src,
> + size);
> + return ret;
> }
>
> static nokprobe_inline int
> diff --git a/mm/maccess.c b/mm/maccess.c
> index cbd9d668aa46e..811f49e8de113 100644
> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -6,36 +6,14 @@
> #include <linux/mm.h>
> #include <linux/uaccess.h>
>
> -static long __probe_kernel_read(void *dst, const void *src, size_t size,
> - bool strict);
> -
> bool __weak probe_kernel_read_allowed(void *dst, const void *unsafe_src,
> - size_t size, bool strict)
> + size_t size)
> {
> return true;
> }
>
> /**
> - * probe_kernel_read(): safely attempt to read from any location
> - * @dst: pointer to the buffer that shall take the data
> - * @src: address to read from
> - * @size: size of the data chunk
> - *
> - * Same as probe_kernel_read_strict() except that for architectures with
> - * not fully separated user and kernel address spaces this function also works
> - * for user address tanges.
> - *
> - * DO NOT USE THIS FUNCTION - it is broken on architectures with entirely
> - * separate kernel and user address spaces, and also a bad idea otherwise.
> - */
> -long probe_kernel_read(void *dst, const void *src, size_t size)
> -{
> - return __probe_kernel_read(dst, src, size, false);
> -}
> -EXPORT_SYMBOL_GPL(probe_kernel_read);
> -
> -/**
> - * probe_kernel_read_strict(): safely attempt to read from kernel-space
> + * probe_kernel_read(): safely attempt to read from kernel-space
> * @dst: pointer to the buffer that shall take the data
> * @src: address to read from
> * @size: size of the data chunk
> @@ -48,18 +26,12 @@ EXPORT_SYMBOL_GPL(probe_kernel_read);
> * probe_kernel_read() suitable for use within regions where the caller
> * already holds mmap_sem, or other locks which nest inside mmap_sem.
> */
> -long probe_kernel_read_strict(void *dst, const void *src, size_t size)
> -{
> - return __probe_kernel_read(dst, src, size, true);
> -}
> -
> -static long __probe_kernel_read(void *dst, const void *src, size_t size,
> - bool strict)
> +long probe_kernel_read(void *dst, const void *src, size_t size)
> {
> long ret;
> mm_segment_t old_fs = get_fs();
>
> - if (!probe_kernel_read_allowed(dst, src, size, strict))
> + if (!probe_kernel_read_allowed(dst, src, size))
> return -EFAULT;
>
> set_fs(KERNEL_DS);
> @@ -73,6 +45,7 @@ static long __probe_kernel_read(void *dst, const void *src, size_t size,
> return -EFAULT;
> return 0;
> }
> +EXPORT_SYMBOL_GPL(probe_kernel_read);
>
> /**
> * probe_user_read(): safely attempt to read from a user-space location
> @@ -180,7 +153,7 @@ long strncpy_from_kernel_unsafe(char *dst, const void *unsafe_addr, long count)
>
> if (unlikely(count <= 0))
> return 0;
> - if (!probe_kernel_read_allowed(dst, unsafe_addr, count, true))
> + if (!probe_kernel_read_allowed(dst, unsafe_addr, count))
> return -EFAULT;
>
> set_fs(KERNEL_DS);
> --
> 2.26.2
>
--
Masami Hiramatsu <mhiramat@...nel.org>
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