From: Dave Martin <Dave.Martin@arm.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: linux-arch@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
Nicolas Pitre <nico@linaro.org>, Tony Lindgren <tony@atomide.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Tyler Baicar <tbaicar@codeaurora.org>,
Will Deacon <will.deacon@arm.com>,
linux-kernel@vger.kernel.org, Oleg Nesterov <oleg@redhat.com>,
James Morse <james.morse@arm.com>,
Al Viro <viro@zeniv.linux.org.uk>,
Olof Johansson <olof@lixom.net>,
Santosh Shilimkar <santosh.shilimkar@ti.com>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 07/11] signal/arm64: Document conflicts with SI_USER and SIGFPE, SIGTRAP, SIGBUS
Date: Tue, 16 Jan 2018 17:24:07 +0000 [thread overview]
Message-ID: <20180116172407.GA22781@e103592.cambridge.arm.com> (raw)
In-Reply-To: <87h8rnox3c.fsf@xmission.com>
On Mon, Jan 15, 2018 at 11:23:03AM -0600, Eric W. Biederman wrote:
> Dave Martin <Dave.Martin@arm.com> writes:
>
> > On Thu, Jan 11, 2018 at 06:59:36PM -0600, Eric W. Biederman wrote:
> >> Setting si_code to 0 results in a userspace seeing an si_code of 0.
> >> This is the same si_code as SI_USER. Posix and common sense requires
> >> that SI_USER not be a signal specific si_code. As such this use of 0
> >> for the si_code is a pretty horribly broken ABI.
> >
> > I think this situation may have come about because 0 is used as a
> > padding value for "impossible" cases -- i.e., things that can't happen
> > unless the kernel is broken, or things that are too unrecoverable for
> > clean error reporting to be helpful.
> >
> > In general, I think these values are not expected to reach userspace in
> > practice.
> >
> > This is not an excuse though -- and not 100% true -- so it's certainly
> > worthy of cleanup.
> >
> >
> > It would be good to approach this similarly for arm and arm64, since
> > the arm64 fault code is derived from arm.
>
> In this case the fault_info is something I have only seen on arm64.
> I have been approaching all architectures the same way.
Bad guess on my part; this table-driven approach seems to be new for
arm64.
> If there is insufficient information without architecture expertise
> to fix this class of error I have been ading FPE_FIXME to them.
Fair enough.
> >> Further use of si_code == 0 guaranteed that copy_siginfo_to_user saw a
> >> value of __SI_KILL and now sees a value of SIL_KILL with the result
> >> that uid and pid fields are copied and which might copying the si_addr
> >> field by accident but certainly not by design. Making this a very
> >> flakey implementation.
> >>
> >> Utilizing FPE_FIXME, BUS_FIXME, TRAP_FIXME siginfo_layout will now return
> >> SIL_FAULT and the appropriate fields will be reliably copied.
> >>
> >> But folks this is a new and unique kind of bad. This is massively
> >> untested code bad. This is inventing new and unique was to get
> >> siginfo wrong bad. This is don't even think about Posix or what
> >> siginfo means bad. This is lots of eyeballs all missing the fact
> >> that the code does the wrong thing bad. This is getting stuck
> >> and keep making the same mistake bad.
> >>
> >> I really hope we can find a non userspace breaking fix for this on a
> >> port as new as arm64.
> >
> >> Possible ABI fixes include:
> >> - Send the signal without siginfo
> >> - Don't generate a signal
> >
> > The above two sould like ABI breaks?
>
> They are ways I have seen code on other platforms deal with
> not information to generate siginfo. Sending the signal without siginfo
> is roughly equivalent to your send SIGKILL suggestion below.
>
> A good example of that is code that calls force_sigsegv.
>
> Calling "force_sig(SIGBUS, current);" is perfectly valid.
> And then the parent when it reaped the process would have
> a little more information to go on when guessing what happened
> to the process.
>
> >> - Possibly assign and use an appropriate si_code
> >> - Don't handle cases which can't happen
> >
> > I think a mixture of these two is the best approach.
> >
> > In any case, si_code == 0 here doesn't seem to have any explicit meaning.
> > I think we can translate all of the arm64 faults to proper si_codes --
> > see my sketch below. Probably means a bit more thought though.
>
> Yes I would be very happy to see that.
>
> > The only counterargument would be if there is software relying on
> > these bogus signal cases getting si_code == 0 for a useful purpose.
> >
> > The main reason I see to check for SI_USER is to allow a process to
> > filter out spurious signals (say, an asynchronous I/O signal for
> > which si_value would be garbage), and to print out diagnostics
> > before (in the case of a well-behaved program) resetting the signal
> > to SIG_DFL and killing itself to report the signal to the waiter.
> >
> > Daemons may be more discerning about who is allowed to signal them,
> > but overloading SIGBUS (say) as an IPC channel sounds like a very odd
> > thing to do. The same probably applies to any signal that has
> > nontrivial metadata.
>
> Agreed. Although I have seen ltp test cases that do crazy things like
> that.
>
> > Have you found software that is impacted by this in practice?
>
> No.
Searching for si_code on codesearch.debian.net, I looked at a few
random hits. Although this is far from exhaustive, I saw no instance
of code that assumes some arch-specific meaning for SI_USER (or 0).
Most code seems to check for signal-specific si_code values before
assuming that signal-specific signifo fields are valid; or for
SI_USER (or si_code <= 0) before assuming that si_uid and si_pid
are valid.
Returning proper values for si_code values in place of "0" would fix
rather than break such cases.
> I don't expect many userspace applications look at siginfo and
> everything I have found is some rare hard to trigger non-x86 case which
> limits the exposure to userspace applications tremendously.
>
> The angle I am coming at all of this from is that the linux kernel code
> that filled out out struct siginfo was not comprehensible or correct.
I think "not comprehensible or correct" is a pretty good summary of
all signal-related code...
> Internal to the kernel it was using a magic value (not exportable to
> userspace) in the upper bits of si_code. That was causing problems for
> signal injection and converting signals from 32bit to 64bit, and from
> 64bit to 32bit.
>
> So I wrote kernel/signal.c:siginfo_layout() to figure out which fields
> of struct siginfo should be sent to userspace. In doing so I discovered
> that using 0 in si_code (aka SI_USER) is ambiguous, and problematic.
>
> Unfortuantely in most of the cases I have spotted using 0 in the si_code
> requires architectural knowledge that I don't currently have to sort
> out. So the best I can do is change si_code from 0 to
> FPE_FIXME/BUS_FIXME/TRAP_FIXME and bring the architecture maintainers
> attention to this area.
>
> One of the problems that results from all of this is that we copy
> unitialized data to userspace. I am slowly unifying and cleaning the
> code up so that the code is simple enough we can be certain we are
> not copying unitialized data to userspace.
>
> With si_coes of FPE_FIXME/BUS_FIXME/TRAP_FIXME I can at least attempt to
> keep the craziness from happening.
>
> My next step is to unify struct siginfo and struct compat_siginfo
> and the functions that copy them to userspace because there are very
> siginficant problems there.
All of which sounds valuable.
> All of that said I like the way you are thinking about fixing these
> issues.
Is it feasible to have a different internal constant for SI_USER?
Then the generic could warn if it sees si_code == 0. If the
special nonzero KERNEL_SI_USER is seen instead, it is silently
translated to SI_USER (0) for userspace. This might help us
track down cases where 0 is passed by accident.
It may not be worth it though: if the affected cases are ones
that happen never or almost never, a runtime BUG_ON() may not be
helpful for tracking them down.
Also, I'm making an assumption that si_code always flows through
some generic code before reaching userspace (maybe untrue).
> >> +++ b/arch/arm64/kernel/fpsimd.c
> >> @@ -867,7 +867,7 @@ asmlinkage void do_fpsimd_acc(unsigned int esr, struct pt_regs *regs)
> >> asmlinkage void do_fpsimd_exc(unsigned int esr, struct pt_regs *regs)
> >> {
> >> siginfo_t info;
> >> - unsigned int si_code = 0;
> >> + unsigned int si_code = FPE_FIXME;
> >>
> >> if (esr & FPEXC_IOF)
> >> si_code = FPE_FLTINV;
> >
> > This 0 can happen for vector operations where the implementation may
> > not be able to report exactly what happened, for example where
> > the implementer didn't want to pay the cost of tracking exactly
> > what went wrong in each lane.
> >
> > However, the FPEXC_* bits can be garbage in such a case rather
> > than being all zero: we should be checking the TFV bit in the ESR here.
> > This may be a bug.
> >
> > Perhaps FPE_FLTINV should be returned in si_code for such cases: it's
> > not otherwise used on arm64 -- invalid instructions would be reported as
> > SIGILL/ILL_ILLOPC instead).
> >
> > Otherwise, we might want to define a new code or arbitrarily pick
> > one of the existing FLT_* since this is really a more benign condition
> > than executing an illegal instruction. Alternatively, treat the
> > fault as spurious and suppress it, but that doesn't feel right either.
>
> I would love to see this sorted out. There is a very similar pattern
> on several different architectures. I suspect if we have a clean
> solution on one architecture the other architectures will be able to use
> that solution as well.
Since user code that relies on checking si_code for fp exceptions will
probably already break in these cases, we can probably fudge things a
bit here.
I'll have a think. IEEE may also define some rules that are relevant
here...
For the proposed conversion of the si_code==0 cases for arm64, I'll
draft an RFC for discussion (hopefully sometime this week).
[...]
Cheers
---Dave
next prev parent reply other threads:[~2018-01-16 17:24 UTC|newest]
Thread overview: 89+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-12 0:57 [PATCH 00/11] siginfo fixes/cleanups esp SI_USER Eric W. Biederman
2018-01-12 0:59 ` [PATCH 01/11] signal: Simplify and fix kdb_send_sig Eric W. Biederman
2018-01-12 0:59 ` [PATCH 02/11] signal/sh: Ensure si_signo is initialized in do_divide_error Eric W. Biederman
2018-01-12 0:59 ` [PATCH 03/11] signal/openrisc: Fix do_unaligned_access to send the proper signal Eric W. Biederman
2018-01-12 13:25 ` Stafford Horne
2018-01-12 17:37 ` Eric W. Biederman
2018-01-12 0:59 ` [PATCH 04/11] signal/parisc: Document a conflict with SI_USER with SIGFPE Eric W. Biederman
2018-01-12 22:29 ` Helge Deller
2018-01-13 21:06 ` Eric W. Biederman
2018-01-14 1:46 ` Eric W. Biederman
2018-02-23 0:15 ` Eric W. Biederman
2018-02-25 19:49 ` Helge Deller
2018-02-27 2:19 ` Eric W. Biederman
2018-01-12 0:59 ` [PATCH 05/11] signal/metag: " Eric W. Biederman
2018-01-12 0:59 ` [PATCH 06/11] signal/powerpc: Document conflicts with SI_USER and SIGFPE and SIGTRAP Eric W. Biederman
2018-01-12 0:59 ` [PATCH 07/11] signal/arm64: Document conflicts with SI_USER and SIGFPE,SIGTRAP,SIGBUS Eric W. Biederman
2018-01-15 16:30 ` [PATCH 07/11] signal/arm64: Document conflicts with SI_USER and SIGFPE, SIGTRAP, SIGBUS Dave Martin
2018-01-15 17:23 ` Eric W. Biederman
2018-01-16 17:24 ` Dave Martin [this message]
2018-01-16 22:28 ` Eric W. Biederman
2018-01-17 11:46 ` Dave Martin
2018-01-17 11:57 ` Russell King - ARM Linux
2018-01-17 12:15 ` Dave Martin
2018-01-17 12:37 ` Russell King - ARM Linux
2018-01-17 15:37 ` Dave Martin
2018-01-17 15:49 ` Russell King - ARM Linux
2018-01-17 16:11 ` Dave Martin
2018-01-17 16:45 ` Eric W. Biederman
2018-01-17 17:14 ` Russell King - ARM Linux
2018-01-24 21:28 ` Eric W. Biederman
2018-01-17 17:17 ` Dave Martin
2018-01-17 17:24 ` Eric W. Biederman
2018-01-17 17:39 ` Dave Martin
2018-01-15 19:30 ` James Morse
2018-01-12 0:59 ` [PATCH 08/11] signal/arm: Document conflicts with SI_USER and SIGFPE Eric W. Biederman
2018-01-15 17:49 ` Russell King - ARM Linux
2018-01-15 20:12 ` Eric W. Biederman
2018-01-16 17:41 ` Dave Martin
2018-01-19 12:05 ` Dave Martin
2018-01-12 0:59 ` [PATCH 09/11] signal: Reduce copy_siginfo to just a memcpy Eric W. Biederman
2018-01-12 0:59 ` [PATCH 10/11] signal: Introduce clear_siginfo Eric W. Biederman
2018-01-12 0:59 ` [PATCH 11/11] signal: Ensure generic siginfos the kernel sends have all bits initialized Eric W. Biederman
2018-01-12 20:29 ` [PATCH 0/2] siginfo fixes Eric W. Biederman
2018-01-12 20:31 ` [PATCH 1/2] mn10300/misalignment: Use SIGSEGV SEGV_MAPERR to report a failed user copy Eric W. Biederman
2018-01-12 20:31 ` [PATCH 2/2] x86/mm/pkeys: Fix fill_sig_info_pkey Eric W. Biederman
2018-01-14 11:44 ` [tip:x86/urgent] " tip-bot for Eric W. Biederman
2018-01-16 0:39 ` [PATCH 00/22] siginfo unification Eric W. Biederman
2018-01-16 0:39 ` [PATCH 01/22] signal: Document all of the signals that use the _sigfault union member Eric W. Biederman
2018-01-16 0:39 ` [PATCH 02/22] signal: Document the strange si_codes used by ptrace event stops Eric W. Biederman
2018-01-16 0:39 ` [PATCH 03/22] signal: Document glibc's si_code of SI_ASYNCNL Eric W. Biederman
2018-01-16 0:39 ` [PATCH 04/22] signal: Ensure no siginfo union member increases the size of struct siginfo Eric W. Biederman
2018-01-16 0:39 ` [PATCH 05/22] signal: Clear si_sys_private before copying siginfo to userspace Eric W. Biederman
2018-01-16 0:39 ` [PATCH 06/22] signal: Remove _sys_private and _overrun_incr from struct compat_siginfo Eric W. Biederman
2018-01-16 0:39 ` [PATCH 07/22] ia64/signal: switch to generic struct siginfo Eric W. Biederman
2018-01-16 0:39 ` [PATCH 08/22] signal/ia64: switch the last arch-specific copy_siginfo_to_user() to generic version Eric W. Biederman
2018-01-16 0:39 ` [PATCH 09/22] signal/mips: switch mips to generic siginfo Eric W. Biederman
2018-01-16 0:39 ` [PATCH 10/22] signal: Remove unnecessary ifdefs now that there is only one struct siginfo Eric W. Biederman
2018-01-16 0:39 ` [PATCH 11/22] signal: kill __ARCH_SI_UID_T Eric W. Biederman
2018-01-16 0:39 ` [PATCH 12/22] signal: unify compat_siginfo_t Eric W. Biederman
2018-01-16 0:40 ` [PATCH 13/22] signal: Move addr_lsb into the _sigfault union for clarity Eric W. Biederman
2018-03-16 19:00 ` Dave Hansen
2018-03-16 19:24 ` Dave Hansen
2018-03-16 20:06 ` Eric W. Biederman
2018-03-16 20:33 ` Dave Hansen
2018-03-16 21:08 ` Eric W. Biederman
2018-01-16 0:40 ` [PATCH 14/22] signal/powerpc: Remove redefinition of NSIGTRAP on powerpc Eric W. Biederman
2018-01-16 0:40 ` [PATCH 15/22] signal/ia64: Move the ia64 specific si_codes to asm-generic/siginfo.h Eric W. Biederman
2018-01-16 0:40 ` [PATCH 16/22] signal/frv: Move the frv " Eric W. Biederman
2018-01-16 0:40 ` [PATCH 17/22] signal/tile: Move the tile " Eric W. Biederman
2018-01-16 0:40 ` [PATCH 18/22] signal/blackfin: Move the blackfin " Eric W. Biederman
2018-01-16 0:40 ` [PATCH 19/22] signal/blackfin: Remove pointless UID16_SIGINFO_COMPAT_NEEDED Eric W. Biederman
2018-01-16 0:40 ` [PATCH 20/22] signal: Unify and correct copy_siginfo_from_user32 Eric W. Biederman
2018-01-16 0:40 ` [PATCH 21/22] signal: Remove the code to clear siginfo before calling copy_siginfo_from_user32 Eric W. Biederman
2018-01-16 0:40 ` [PATCH 22/22] signal: Unify and correct copy_siginfo_to_user32 Eric W. Biederman
2018-01-19 18:03 ` Al Viro
2018-01-19 21:04 ` Eric W. Biederman
2018-01-23 21:05 ` [PATCH 00/10] siginfo infrastructure Eric W. Biederman
2018-01-23 21:07 ` [PATCH 01/10] ptrace: Use copy_siginfo in setsiginfo and getsiginfo Eric W. Biederman
2018-01-23 21:07 ` [PATCH 02/10] signal/arm64: Better isolate the COMPAT_TASK portion of ptrace_hbptriggered Eric W. Biederman
2018-01-23 21:07 ` [PATCH 03/10] signal: Don't use structure initializers for struct siginfo Eric W. Biederman
2018-01-23 21:07 ` [PATCH 04/10] signal: Replace memset(info,...) with clear_siginfo for clarity Eric W. Biederman
2018-01-23 21:07 ` [PATCH 05/10] signal: Add send_sig_fault and force_sig_fault Eric W. Biederman
2018-01-23 21:07 ` [PATCH 06/10] signal: Helpers for faults with specialized siginfo layouts Eric W. Biederman
2018-01-24 19:26 ` Ram Pai
2018-01-24 20:54 ` Eric W. Biederman
2018-01-23 21:07 ` [PATCH 07/10] signal/powerpc: Remove unnecessary signal_code parameter of do_send_trap Eric W. Biederman
2018-01-23 21:07 ` [PATCH 08/10] signal/ptrace: Add force_sig_ptrace_errno_trap and use it where needed Eric W. Biederman
2018-01-23 21:07 ` [PATCH 09/10] mm/memory_failure: Remove unused trapno from memory_failure Eric W. Biederman
2018-01-23 21:07 ` [PATCH 10/10] signal/memory-failure: Use force_sig_mceerr and send_sig_mceerr Eric W. Biederman
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