| Age | Commit message (Collapse) | Author |
|---|
|
TIF_NOTIFY_RESUME is a multiplexing TIF bit, which is suboptimal especially
with the RSEQ fast path depending on it, but not really handling it.
Define a separate TIF_RSEQ in the generic TIF space and enable the full
separation of fast and slow path for architectures which utilize that.
That avoids the hassle with invocations of resume_user_mode_work() from
hypervisors, which clear TIF_NOTIFY_RESUME. It makes the therefore required
re-evaluation at the end of vcpu_run() a NOOP on architectures which
utilize the generic TIF space and have a separate TIF_RSEQ.
The hypervisor TIF handling does not include the separate TIF_RSEQ as there
is no point in doing so. The guest does neither know nor care about the VMM
host applications RSEQ state. That state is only relevant when the ioctl()
returns to user space.
The fastpath implementation still utilizes TIF_NOTIFY_RESUME for failure
handling, but this only happens within exit_to_user_mode_loop(), so
arguably the hypervisor ioctl() code is long done when this happens.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.903622031@linutronix.de
|
|
Separate the interrupt and syscall exit handling. Syscall exit does not
require to clear the user_irq bit as it can't be set. On interrupt exit it
can be set when the interrupt did not result in a scheduling event and
therefore the return path did not invoke the TIF work handling, which would
have cleared it.
The debug check for the event state is also not really required even when
debug mode is enabled via the static key. Debug mode is largely aiding user
space by enabling a larger amount of validation checks, which cause a
segfault when a malformed critical section is detected. In production mode
the critical section handling takes the content mostly as is and lets user
space keep the pieces when it screwed up.
On kernel changes in that area the state check is useful, but that can be
done when lockdep is enabled, which is anyway a required test scenario for
fundamental changes.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.842785700@linutronix.de
|
|
Implement the actual logic for handling RSEQ updates in a fast path after
handling the TIF work and at the point where the task is actually returning
to user space.
This is the right point to do that because at this point the CPU and the MM
CID are stable and cannot longer change due to yet another reschedule.
That happens when the task is handling it via TIF_NOTIFY_RESUME in
resume_user_mode_work(), which is invoked from the exit to user mode work
loop.
The function is invoked after the TIF work is handled and runs with
interrupts disabled, which means it cannot resolve page faults. It
therefore disables page faults and in case the access to the user space
memory faults, it:
- notes the fail in the event struct
- raises TIF_NOTIFY_RESUME
- returns false to the caller
The caller has to go back to the TIF work, which runs with interrupts
enabled and therefore can resolve the page faults. This happens mostly on
fork() when the memory is marked COW.
If the user memory inspection finds invalid data, the function returns
false as well and sets the fatal flag in the event struct along with
TIF_NOTIFY_RESUME. The slow path notify handler has to evaluate that flag
and terminate the task with SIGSEGV as documented.
The initial decision to invoke any of this is based on one flags in the
event struct: @sched_switch. The decision is in pseudo ASM:
load tsk::event::sched_switch
jnz inspect_user_space
mov $0, tsk::event::events
...
leave
So for the common case where the task was not scheduled out, this really
boils down to three instructions before going out if the compiler is not
completely stupid (and yes, some of them are).
If the condition is true, then it checks, whether CPU ID or MM CID have
changed. If so, then the CPU/MM IDs have to be updated and are thereby
cached for the next round. The update unconditionally retrieves the user
space critical section address to spare another user*begin/end() pair. If
that's not zero and tsk::event::user_irq is set, then the critical section
is analyzed and acted upon. If either zero or the entry came via syscall
the critical section analysis is skipped.
If the comparison is false then the critical section has to be analyzed
because the event flag is then only true when entry from user was by
interrupt.
This is provided without the actual hookup to let reviewers focus on the
implementation details. The hookup happens in the next step.
Note: As with quite some other optimizations this depends on the generic
entry infrastructure and is not enabled to be sucked into random
architecture implementations.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.638929615@linutronix.de
|
|
Replace the whole logic with a new implementation, which is shared with
signal delivery and the upcoming exit fast path.
Contrary to the original implementation, this ignores invocations from
KVM/IO-uring, which invoke resume_user_mode_work() with the @regs argument
set to NULL.
The original implementation updated the CPU/Node/MM CID fields, but that
was just a side effect, which was addressing the problem that this
invocation cleared TIF_NOTIFY_RESUME, which in turn could cause an update
on return to user space to be lost.
This problem has been addressed differently, so that it's not longer
required to do that update before entering the guest.
That might be considered a user visible change, when the hosts thread TLS
memory is mapped into the guest, but as this was never intentionally
supported, this abuse of kernel internal implementation details is not
considered an ABI break.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.517640811@linutronix.de
|
|
Provide a new and straight forward implementation to set the IDs (CPU ID,
Node ID and MM CID), which can be later inlined into the fast path.
It does all operations in one scoped_user_rw_access() section and retrieves
also the critical section member (rseq::cs_rseq) from user space to avoid
another user..begin/end() pair. This is in preparation for optimizing the
fast path to avoid extra work when not required.
On rseq registration set the CPU ID fields to RSEQ_CPU_ID_UNINITIALIZED and
node and MM CID to zero. That's the same as the kernel internal reset
values. That makes the debug validation in the exit code work correctly on
the first exit to user space.
Use it to replace the whole related zoo in rseq.c
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.393972266@linutronix.de
|
|
Make the syscall exit debug mechanism available via the static branch on
architectures which utilize the generic entry code.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.333440475@linutronix.de
|
|
Disconnect it from the config switch and use the static debug branch. This
is a temporary measure for validating the rework. At the end this check
needs to be hidden behind lockdep as it has nothing to do with the other
debug infrastructure, which mainly aids user space debugging by enabling a
zoo of checks which terminate misbehaving tasks instead of letting them
keep the hard to diagnose pieces.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.272660745@linutronix.de
|
|
Provide a straight forward implementation to check for and eventually
clear/fixup critical sections in user space.
The non-debug version does only the minimal sanity checks and aims for
efficiency.
There are two attack vectors, which are checked for:
1) An abort IP which is in the kernel address space. That would cause at
least x86 to return to kernel space via IRET.
2) A rogue critical section descriptor with an abort IP pointing to some
arbitrary address, which is not preceded by the RSEQ signature.
If the section descriptors are invalid then the resulting misbehaviour of
the user space application is not the kernels problem.
The kernel provides a run-time switchable debug slow path, which implements
the full zoo of checks including termination of the task when one of the
gazillion conditions is not met.
Replace the zoo in rseq.c with it and invoke it from the TIF_NOTIFY_RESUME
handler. Move the remainders into the CONFIG_DEBUG_RSEQ section, which will
be replaced and removed in a subsequent step.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.151465632@linutronix.de
|
|
Config based debug is rarely turned on and is not available easily when
things go wrong.
Provide a static branch to allow permanent integration of debug mechanisms
along with the usual toggles in Kconfig, command line and debugfs.
Requested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.089270547@linutronix.de
|
|
Analyzing the call frequency without actually using tracing is helpful for
analysis of this infrastructure. The overhead is minimal as it just
increments a per CPU counter associated to each operation.
The debugfs readout provides a racy sum of all counters.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084307.027916598@linutronix.de
|
|
Provide tracepoint wrappers for the upcoming RSEQ exit to user space inline
fast path, so that the header can be safely included by code which defines
actual trace points.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084306.967114316@linutronix.de
|
|
For RSEQ the only relevant reason to inspect and eventually fixup (abort)
user space critical sections is when user space was interrupted and the
task was scheduled out.
If the user to kernel entry was from a syscall no fixup is required. If
user space invokes a syscall from a critical section it can keep the
pieces as documented.
This is only supported on architectures which utilize the generic entry
code. If your architecture does not use it, bad luck.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084306.905067101@linutronix.de
|
