Subject: Re: [PATCH v12 7/8] signal: define the field siginfo.si_xflags

From: Dave Martin <[email-masked]>
Date: Mon, 2 Nov 2020 17:37:52 +0000

On Fri, Oct 16, 2020 at 05:12:32PM -0700, Peter Collingbourne wrote:
> This field will contain flags that may be used by signal handlers to
> determine whether other fields in the _sigfault portion of siginfo are
> valid. An example use case is the following patch, which introduces
> the si_addr_tag_bits{,_mask} fields.
> 
> A new sigcontext flag, SA_XFLAGS, is introduced in order to allow
> a signal handler to require the kernel to set the field (but note
> that the field will be set anyway if the kernel supports the flag,
> regardless of its value). In combination with the previous patches,
> this allows a userspace program to determine whether the kernel will
> set the field.

Apologies for this coming rather late:

It occurs to me that we might want a more specific name, since this only
applies to fault signals -- say, SA_FAULTFLAGS.

If we end up wanting to add flags fields for other signal types, then we
might end up needing a SA_ flag for each, which would be a bit annoying.

So, alternatively. I wonder whether it's worth preemptively adding an
extra flags to every kind of kernel-generated siginfo.  If so, then
having a single SA_XFLAGS would be fine.

If added flags fields all over the place is considered overkill, then I
guess it's sufficient to rename this flag.

If renaming, the actual flags field in siginfo should also be renamed to
match.

> 
> It is possible for an si_xflags-unaware program to cause a signal
> handler in an si_xflags-aware program to be called with a provided
> siginfo data structure by using one of the following syscalls:
> 
> - ptrace(PTRACE_SETSIGINFO)
> - pidfd_send_signal
> - rt_sigqueueinfo
> - rt_tgsigqueueinfo
> 
> So we need to prevent the si_xflags-unaware program from causing an
> uninitialized read of si_xflags in the si_xflags-aware program when
> it uses one of these syscalls.
> 
> The last three cases can be handled by observing that each of these
> syscalls fails if si_code >= 0. We also observe that kill(2) and
> tgkill(2) may be used to send a signal where si_code == 0 (SI_USER),
> so we define si_xflags to only be valid if si_code > 0.
> 
> There is no such check on si_code in ptrace(PTRACE_SETSIGINFO), so
> we make ptrace(PTRACE_SETSIGINFO) clear the si_xflags field if it
> detects that the signal would use the _sigfault layout, and introduce
> a new ptrace request type, PTRACE_SETSIGINFO2, that a si_xflags-aware
> program may use to opt out of this behavior.
> 
> It is also possible for the kernel to inject a signal specified to
> use _sigfault by calling force_sig (e.g. there are numerous calls to
> force_sig(SIGSEGV)). In this case si_code is set to SI_KERNEL and the
> _kill union member is used, so document that si_code must be < SI_KERNEL.
> 
> Ideally this field could have just been named si_flags, but that
> name was already taken by ia64, so a different name was chosen.
> 
> I considered making ia64's si_flags a generic field and having it
> appear at the end of _sigfault (in the same place as this patch has
> si_xflags) on non-ia64, keeping it in the same place on ia64. ia64's
> si_flags is a 32-bit field with only one flag bit allocated, so we
> would have 31 bits to use if we do this. However, it seems simplest
> to avoid entangling these fields.
> 
> Signed-off-by: Peter Collingbourne <[email-masked]>
> Link: https://linux-review.googlesource.com/id/Ide155ce29366c3eab2a944ae4c51205982e5b8b2
> ---
> v12:
> - Change type of si_xflags to u32 to avoid increasing alignment
> - Add si_xflags to signal_compat.c test cases
> 
> v11:
> - update comment to say that si_code must > 0
> - change ptrace(PTRACE_SETSIGINFO2) to take a flags argument
> 
> v10:
> - make the new field compatible with the various ways
>   that a siginfo can be injected from another process
> - eliminate some duplication by adding a refactoring patch
>   before this one
> 
>  arch/powerpc/platforms/powernv/vas-fault.c |  1 +
>  arch/x86/kernel/signal_compat.c            |  7 +++--
>  include/linux/compat.h                     |  2 ++
>  include/linux/signal_types.h               |  2 +-
>  include/uapi/asm-generic/siginfo.h         |  4 +++
>  include/uapi/asm-generic/signal-defs.h     |  4 +++
>  include/uapi/linux/ptrace.h                | 12 ++++++++
>  kernel/ptrace.c                            | 32 ++++++++++++++++++----
>  kernel/signal.c                            |  3 ++
>  9 files changed, 58 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/powernv/vas-fault.c b/arch/powerpc/platforms/powernv/vas-fault.c
> index 3d21fce254b7..3bbb335561f5 100644
> --- a/arch/powerpc/platforms/powernv/vas-fault.c
> +++ b/arch/powerpc/platforms/powernv/vas-fault.c
> @@ -154,6 +154,7 @@ static void update_csb(struct vas_window *window,
>  	info.si_errno = EFAULT;
>  	info.si_code = SEGV_MAPERR;
>  	info.si_addr = csb_addr;
> +	info.si_xflags = 0;
>  
>  	/*
>  	 * process will be polling on csb.flags after request is sent to
> diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
> index ddfd919be46c..243a8cc3b41b 100644
> --- a/arch/x86/kernel/signal_compat.c
> +++ b/arch/x86/kernel/signal_compat.c
> @@ -121,8 +121,8 @@ static inline void signal_compat_build_tests(void)
>  #endif
>  
>  	CHECK_CSI_OFFSET(_sigfault);
> -	CHECK_CSI_SIZE  (_sigfault, 4*sizeof(int));
> -	CHECK_SI_SIZE   (_sigfault, 8*sizeof(int));
> +	CHECK_CSI_SIZE  (_sigfault, 8*sizeof(int));
> +	CHECK_SI_SIZE   (_sigfault, 16*sizeof(int));
>  
>  	BUILD_BUG_ON(offsetof(siginfo_t, si_addr) != 0x10);
>  	BUILD_BUG_ON(offsetof(compat_siginfo_t, si_addr) != 0x0C);
> @@ -138,6 +138,9 @@ static inline void signal_compat_build_tests(void)
>  	BUILD_BUG_ON(offsetof(siginfo_t, si_pkey) != 0x20);
>  	BUILD_BUG_ON(offsetof(compat_siginfo_t, si_pkey) != 0x14);
>  
> +	BUILD_BUG_ON(offsetof(siginfo_t, si_xflags) != 0x48);
> +	BUILD_BUG_ON(offsetof(compat_siginfo_t, si_xflags) != 0x28);
> +
>  	CHECK_CSI_OFFSET(_sigpoll);
>  	CHECK_CSI_SIZE  (_sigpoll, 2*sizeof(int));
>  	CHECK_SI_SIZE   (_sigpoll, 4*sizeof(int));
> diff --git a/include/linux/compat.h b/include/linux/compat.h
> index 14d514233e1d..ea77a24ce6a2 100644
> --- a/include/linux/compat.h
> +++ b/include/linux/compat.h
> @@ -236,7 +236,9 @@ typedef struct compat_siginfo {
>  					char _dummy_pkey[__COMPAT_ADDR_BND_PKEY_PAD];
>  					u32 _pkey;
>  				} _addr_pkey;
> +				compat_uptr_t _pad[6];
>  			};
> +			u32 _xflags;
>  		} _sigfault;
>  
>  		/* SIGPOLL */
> diff --git a/include/linux/signal_types.h b/include/linux/signal_types.h
> index a7887ad84d36..75ca861d982a 100644
> --- a/include/linux/signal_types.h
> +++ b/include/linux/signal_types.h
> @@ -78,6 +78,6 @@ struct ksignal {
>  
>  #define UAPI_SA_FLAGS                                                          \
>  	(SA_NOCLDSTOP | SA_NOCLDWAIT | SA_SIGINFO | SA_ONSTACK | SA_RESTART |  \
> -	 SA_NODEFER | SA_RESETHAND | __ARCH_UAPI_SA_FLAGS)
> +	 SA_NODEFER | SA_RESETHAND | SA_XFLAGS | __ARCH_UAPI_SA_FLAGS)
>  
>  #endif /* _LINUX_SIGNAL_TYPES_H */
> diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
> index 7aacf9389010..8158d5df666a 100644
> --- a/include/uapi/asm-generic/siginfo.h
> +++ b/include/uapi/asm-generic/siginfo.h
> @@ -91,7 +91,9 @@ union __sifields {
>  				char _dummy_pkey[__ADDR_BND_PKEY_PAD];
>  				__u32 _pkey;
>  			} _addr_pkey;
> +			void *_pad[6];
>  		};
> +		__u32 _xflags;
>  	} _sigfault;
>  
>  	/* SIGPOLL */
> @@ -152,6 +154,8 @@ typedef struct siginfo {
>  #define si_trapno	_sifields._sigfault._trapno
>  #endif
>  #define si_addr_lsb	_sifields._sigfault._addr_lsb
> +/* si_xflags is only valid if 0 < si_code < SI_KERNEL */
> +#define si_xflags	_sifields._sigfault._xflags
>  #define si_lower	_sifields._sigfault._addr_bnd._lower
>  #define si_upper	_sifields._sigfault._addr_bnd._upper
>  #define si_pkey		_sifields._sigfault._addr_pkey._pkey
> diff --git a/include/uapi/asm-generic/signal-defs.h b/include/uapi/asm-generic/signal-defs.h
> index 0126ebda4d31..cd522819f4ba 100644
> --- a/include/uapi/asm-generic/signal-defs.h
> +++ b/include/uapi/asm-generic/signal-defs.h
> @@ -20,6 +20,9 @@
>   * so this bit allows flag bit support to be detected from userspace while
>   * allowing an old kernel to be distinguished from a kernel that supports every
>   * flag bit.
> + * SA_XFLAGS indicates that the signal handler requires the siginfo.si_xflags
> + * field to be valid. Note that if the kernel supports SA_XFLAGS, the field will
> + * be valid regardless of the value of this flag.
>   *
>   * SA_ONESHOT and SA_NOMASK are the historical Linux names for the Single
>   * Unix names RESETHAND and NODEFER respectively.
> @@ -49,6 +52,7 @@
>  #define SA_RESETHAND	0x80000000
>  #endif
>  #define SA_UNSUPPORTED	0x00000400
> +#define SA_XFLAGS	0x00000800
>  
>  #define SA_NOMASK	SA_NODEFER
>  #define SA_ONESHOT	SA_RESETHAND
> diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
> index a71b6e3b03eb..93946edf0139 100644
> --- a/include/uapi/linux/ptrace.h
> +++ b/include/uapi/linux/ptrace.h
> @@ -101,6 +101,18 @@ struct ptrace_syscall_info {
>  	};
>  };
>  
> +#define PTRACE_SETSIGINFO2	0x420f
> +/*
> + * These flags are passed as the addr argument to ptrace.
> + */
> +
> +/*
> + * Asserts that the caller is aware of the field siginfo.si_xflags. Prevents
> + * the kernel from automatically setting the field to 0 when the signal uses
> + * a sigfault layout.
> + */
> +#define PTRACE_SIGINFO_XFLAGS	0x1
> +
>  /*
>   * These values are stored in task->ptrace_message
>   * by tracehook_report_syscall_* to describe the current syscall-stop.
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 43d6179508d6..85b5b4e38661 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -687,18 +687,32 @@ static int ptrace_getsiginfo(struct task_struct *child, kernel_siginfo_t *info)
>  	return error;
>  }
>  
> -static int ptrace_setsiginfo(struct task_struct *child, const kernel_siginfo_t *info)
> +static int ptrace_setsiginfo(struct task_struct *child, unsigned long flags,
> +			     kernel_siginfo_t *info)
>  {
> -	unsigned long flags;
> +	unsigned long lock_flags;
>  	int error = -ESRCH;
>  
> -	if (lock_task_sighand(child, &flags)) {
> +	if (flags & ~PTRACE_SIGINFO_XFLAGS) {
> +		return -EINVAL;
> +	}
> +
> +	/*
> +	 * If the caller is unaware of si_xflags and we're using a layout that
> +	 * requires it, set it to 0 which means "no fields are available".
> +	 */
> +	if (!(flags & PTRACE_SIGINFO_XFLAGS) &&
> +	    siginfo_layout_is_fault(
> +		    siginfo_layout(info->si_signo, info->si_code)))
> +		info->si_xflags = 0;
> +
> +	if (lock_task_sighand(child, &lock_flags)) {
>  		error = -EINVAL;
>  		if (likely(child->last_siginfo != NULL)) {
>  			copy_siginfo(child->last_siginfo, info);
>  			error = 0;
>  		}
> -		unlock_task_sighand(child, &flags);
> +		unlock_task_sighand(child, &lock_flags);
>  	}
>  	return error;
>  }
> @@ -1038,9 +1052,12 @@ int ptrace_request(struct task_struct *child, long request,
>  		break;
>  
>  	case PTRACE_SETSIGINFO:
> +		addr = 0;

If this is intended to fall through, please add a

		/* fall through */

comment here (newer GCC has warnings to catch this; not sure about
clang, but I'd be surprised if no version warns).

[...]

Otherwise, the patch looks reasonable, but I'll wait for your response
on the above points before reviewing in detail.

Cheers
---Dave

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