Re: apparent race condition in mttcg memory handling

[Gustavo Romero <gustavo.romero@linaro.org>]

Hi Michael,

On 5/30/25 16:20, Michael Tokarev wrote:
> Hi!
> 
> For quite some time (almost whole day yesterday) I'm trying to find out
> what's going on with mmtcg in qemu.  There's apparently a race condition
> somewhere, like a use-after-free or something.
> 
> It started as an incarnation of
> https://gitlab.com/qemu-project/qemu/-/issues/1921 -- the same assertion
> failure, but on an x86_64 host this time (it's also mentioned in that
> issue).
> 
> However, that particular assertion failure is not the only possible
> outcome.  We're hitting multiple assertion failures or SIGSEGVs in
> physmem.c and related files, - 4 or 5 different places so far.
> 
> The problem here is that the bug is rather difficult to reproduce.
> What I've been using so far was to make most host cores busy, and
> specify amount of virtual CPUs close to actual host cores (threads).
> 
> For example, on my 4-core, 8-threads notebook, I used `stress -c 8`
> and ran qemu with -smp 10, to trigger this issue. However, on this
> very notebook it is really difficult to trigger it, - it happens
> every 30..50 runs or so.
> 
> The reproducer I was using - it was just booting kernel, no user-
> space is needed. Qemu crashes during kernel init, or it runs fine.
> 
> I used regular kernel from debian sid:
>   http://deb.debian.org/debian/pool/main/l/linux-signed-amd64/linux-image-amd64_6.12.29-1_amd64.deb
> Extract vmlinuz-6.12.29-amd64 from there.

The link above is broken. Googling for "linux-image-amd64_6.12.29-1_amd64.deb" successful too.

Could you point out another image to reproduce it?


Cheers,
Gustavo

> In order to simplify the reproducing, I created a tiny initrd with
> just one executable in there, which does a poweroff:
> 
> cat >poweroff.c <<'EOF'
> #include <sys/reboot.h>
> #include <unistd.h>
> 
> int main(void) {
>    reboot(RB_POWER_OFF);
>    sleep(5);
>    return 0;
> }
> EOF
> diet gcc -static -o init poweroff.c
> echo init | cpio -o -H newc > initrd
> 
> (it uses dietlibc, optional, just to make the initrd smaller).
> 
> Now, the qemu invocation I used:
> 
>   qemu-system-x86_64 -kernel vmlinuz -initrd initrd \
>     -append "console=ttyS0" \
>     -vga none -display none \
>     -serial file:/dev/tty \
>     -monitor stdio \
>     -m 256 \
>     -smp 16
> 
> This way, it either succeeds, terminating normally due to
> the initrd hating the system, or it will segfault or assert
> as per the issue.
> 
> For a 64-core machine, I used -smp 64, and had 16..40 cores
> being busy with other stuff.  Also, adding `nice' in front
> of that command apparently helps.
> 
> Now, to the various issues/places I've hit.  Here's a typical
> output:
> 
> ...
> [    3.129806] smpboot: x86: Booting SMP configuration:
> [    3.135789] .... node  #0, CPUs:        #1  #2  #3  #4  #5  #6  #7 #8  #9
> [    0.000000] calibrate_delay_direct() failed to get a good estimate for loops_per_jiffy.
> [    0.000000] Probably due to long platform interrupts. Consider using "lpj=" boot option.
> [    0.000000] calibrate_delay_direct() failed to get a good estimate for loops_per_jiffy.
> [    0.000000] Probably due to long platform interrupts. Consider using "lpj=" boot option.
> [    0.000000] calibrate_delay_direct() failed to get a good estimate for loops_per_jiffy.
> [    0.000000] Probably due to long platform interrupts. Consider using "lpj=" boot option.
> [    0.000000] calibrate_delay_direct() failed to get a good estimate for loops_per_jiffy.
> [    0.000000] Probably due to long platform interrupts. Consider using "lpj=" boot option.
> [    4.494389] calibrate_delay_direct() failed to get a good estimate for loops_per_jiffy.
> [    4.494389] Probably due to long platform interrupts. Consider using "lpj=" boot option.
> [    4.494396] calibrate_delay_direct() failed to get a good estimate for loops_per_jiffy.
> [    4.494396] Probably due to long platform interrupts. Consider using "lpj=" boot option.
> [    4.494401] calibrate_delay_direct() failed to get a good estimate for loops_per_jiffy.
> [    4.494401] Probably due to long platform interrupts. Consider using "lpj=" boot option.
> [    4.494408] calibrate_delay_direct() failed to get a good estimate for loops_per_jiffy.
> [    4.494408] Probably due to long platform interrupts. Consider using "lpj=" boot option.
> [    4.494415] calibrate_delay_direct() failed to get a good estimate for loops_per_jiffy.
> [    4.494415] Probably due to long platform interrupts. Consider using "lpj=" boot option.
> [    5.864038] smp: Brought up 1 node, 10 CPUs
> [    5.865772] smpboot: Total of 10 processors activated (25983.25 BogoMIPS)
> [    6.119683] Memory: 200320K/261624K available (16384K kernel code, 2486K rwdata, 11780K rodata, 4148K init, 4956K bss, 53176K reserved, 0K cma-reserved)
> [    6.591933] devtmpfs: initialized
> [    6.635844] x86/mm: Memory block size: 128MB
> [    6.756849] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645041785100000 ns
> [    6.774545] futex hash table entries: 4096 (order: 6, 262144 bytes, linear)
> [    6.840775] pinctrl core: initialized pinctrl subsystem
> [    7.117085] NET: Registered PF_NETLINK/PF_ROUTE protocol family
> [    7.165883] DMA: preallocated 128 KiB GFP_KERNEL pool for atomic allocations
> [    7.184243] DMA: preallocated 128 KiB GFP_KERNEL|GFP_DMA pool for atomic allocations
> [    7.188322] DMA: preallocated 128 KiB GFP_KERNEL|GFP_DMA32 pool for atomic allocations
> [    7.195902] audit: initializing netlink subsys (disabled)
> [    7.223865] audit: type=2000 audit(1748628013.324:1): state=initialized audit_enabled=0 res=1
> [    7.290904] thermal_sys: Registered thermal governor 'fair_share'
> [    7.291980] thermal_sys: Registered thermal governor 'bang_bang'
> [    7.295875] thermal_sys: Registered thermal governor 'step_wise'
> [    7.299817] thermal_sys: Registered thermal governor 'user_space'
> [    7.303804] thermal_sys: Registered thermal governor 'power_allocator'
> [    7.316281] cpuidle: using governor ladder
> [    7.331907] cpuidle: using governor menu
> [    7.348199] acpiphp: ACPI Hot Plug PCI Controller Driver version: 0.5
> [    7.407802] PCI: Using configuration type 1 for base access
> [    7.417386] mtrr: your CPUs had inconsistent fixed MTRR settings
> [    7.418244] mtrr: your CPUs had inconsistent variable MTRR settings
> [    7.419048] mtrr: your CPUs had inconsistent MTRRdefType settings
> [    7.419938] mtrr: probably your BIOS does not setup all CPUs.
> [    7.420691] mtrr: corrected configuration.
> [    7.461270] kprobes: kprobe jump-optimization is enabled. All kprobes are optimized if possible.
> [    7.591938] HugeTLB: registered 2.00 MiB page size, pre-allocated 0 pages
> [    7.595986] HugeTLB: 28 KiB vmemmap can be freed for a 2.00 MiB page
> [    7.816900] ACPI: Added _OSI(Module Device)
> [    7.819950] ACPI: Added _OSI(Processor Device)
> [    7.823873] ACPI: Added _OSI(3.0 _SCP Extensions)
> [    7.827683] ACPI: Added _OSI(Processor Aggregator Device)
> [    8.000944] ACPI: 1 ACPI AML tables successfully acquired and loaded
> [    8.355952] ACPI: Interpreter enabled
> [    8.406604] ACPI: PM: (supports S0 S3 S4 S5)
> [    8.416143] ACPI: Using IOAPIC for interrupt routing
> [    8.448173] PCI: Using host bridge windows from ACPI; if necessary, use "pci=nocrs" and report a bug
> [    8.468051] PCI: Using E820 reservations for host bridge windows
> [    8.562534] ACPI: Enabled 2 GPEs in block 00 to 0F
> [    9.153432] ACPI: PCI Root Bridge [PCI0] (domain 0000 [bus 00-ff])
> [    9.166585] acpi PNP0A03:00: _OSC: OS supports [ASPM ClockPM Segments MSI HPX-Type3]
> [    9.168452] acpi PNP0A03:00: _OSC: not requesting OS control; OS requires [ExtendedConfig ASPM ClockPM MSI]
> [    9.181933] acpi PNP0A03:00: fail to add MMCONFIG information, can't access extended configuration space under this bridge
> [    9.297562] acpiphp: Slot [2] registered
> ...
> [    9.369007] PCI host bridge to bus 0000:00
> [    9.376590] pci_bus 0000:00: root bus resource [io  0x0000-0x0cf7 window]
> [    9.379987] pci_bus 0000:00: root bus resource [io  0x0d00-0xffff window]
> [    9.383826] pci_bus 0000:00: root bus resource [mem 0x000a0000-0x000bffff window]
> [    9.387818] pci_bus 0000:00: root bus resource [mem 0x10000000-0xfebfffff window]
> [    9.393681] pci_bus 0000:00: root bus resource [mem 0x100000000-0x17fffffff window]
> [    9.396987] pci_bus 0000:00: root bus resource [bus 00-ff]
> [    9.414378] pci 0000:00:00.0: [8086:1237] type 00 class 0x060000 conventional PCI endpoint
> [    9.477179] pci 0000:00:01.0: [8086:7000] type 00 class 0x060100 conventional PCI endpoint
> [    9.494836] pci 0000:00:01.1: [8086:7010] type 00 class 0x010180 conventional PCI endpoint
> [    9.527173] pci 0000:00:01.1: BAR 4 [io  0xc040-0xc04f]
> Segmentation fault
> 
> 
> So it breaks somewhere in PCI init, after SMP/CPUs has been inited
> by the guest kernel.
> 
> Thread 21 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
> 0x0000555555e2e9c0 in section_covers_addr (section=0x7fff58307, addr=182591488) at ../system/physmem.c:309
> 309         return int128_gethi(section->size) ||
> (gdb) p *section
> Cannot access memory at address 0x7fff58307
> 
> This one has been seen multiple times.
> 
> Thread 53 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
> [Switching to Thread 0x7ffe8a7fc6c0 (LWP 104067)]
> 0x0000555555e30382 in memory_region_section_get_iotlb (cpu=0x5555584e0a90, section=0x7fff58c3eac0) at
>               ../system/physmem.c:1002
> 1002        return section - d->map.sections;
> d is NULL here
> 
> 
> Thread 22 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
> [Switching to Thread 0x7fff0bfff6c0 (LWP 57595)]
> 0x0000555555e42c9a in memory_region_get_iommu (mr=0xffffffc1ffffffc1) at include/exec/memory.h:1756
> 1756        if (mr->alias) {
> (gdb) p *mr
> Cannot access memory at address 0xffffffc1ffffffc1
> (gdb) frame 1
> #1  0x0000555555e42cb9 in memory_region_get_iommu (mr=0x7fff54239a10) at include/exec/memory.h:1757
> 1757            return memory_region_get_iommu(mr->alias);
> (gdb) p mr
> $1 = (MemoryRegion *) 0x7fff54239a10
> 
> 
> [    9.222531] pci 0000:00:02.0: BAR 0 [mem 0xfebc0000-0xfebdffff]
> [
> Thread 54 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
> [Switching to Thread 0x7ffebeffd6c0 (LWP 14977)]
> 
> (gdb) l
> 1004    /* Called from RCU critical section */
> 1005    hwaddr memory_region_section_get_iotlb(CPUState *cpu,
> 1006                                           MemoryRegionSection *section)
> 1007    {
> 1008        AddressSpaceDispatch *d = flatview_to_dispatch(section->fv);
> 1009        return section - d->map.sections;
> 1010    }
> 1011
> 1012    static int subpage_register(subpage_t *mmio, uint32_t start, uint32_t end,
> 1013                                uint16_t section);
> 
> (gdb) p *section
> $1 = {size = 4940204083636081308795136, mr = 0x7fff98739760, fv = 0x7fff998f6fd0,
>    offset_within_region = 0, offset_within_address_space = 0, readonly = false,
>    nonvolatile = false, unmergeable = 12}
> 
> (gdb) p *section->fv
> $2 = {rcu = {next = 0x0, func = 0x20281}, ref = 2555275280, ranges = 0x7fff99486a60, nr = 0,
>    nr_allocated = 0, dispatch = 0x0, root = 0xffffffc1ffffffc1}
> 
> (gdb) bt
> #0  0x0000555555e5118c in memory_region_section_get_iotlb (cpu=cpu@entry=0x55555894fdf0,
>      section=section@entry=0x7fff984e6810) at system/physmem.c:1009
> #1  0x0000555555e6e07a in tlb_set_page_full (cpu=cpu@entry=0x55555894fdf0,
>      mmu_idx=mmu_idx@entry=6, addr=addr@entry=151134208, full=full@entry=0x7ffebeffbd60)
>      at accel/tcg/cputlb.c:1088
> #2  0x0000555555e70a92 in tlb_set_page_with_attrs (cpu=cpu@entry=0x55555894fdf0,
>      addr=addr@entry=151134208, paddr=paddr@entry=151134208, attrs=..., prot=<optimized out>,
>      mmu_idx=mmu_idx@entry=6, size=4096) at accel/tcg/cputlb.c:1193
> #3  0x0000555555d4ae44 in x86_cpu_tlb_fill (cs=0x55555894fdf0, addr=151138272,
>      size=<optimized out>, access_type=MMU_DATA_STORE, mmu_idx=6, probe=<optimized out>,
>      retaddr=0) at target/i386/tcg/system/excp_helper.c:624
> #4  0x0000555555e6e8cf in tlb_fill_align (cpu=0x55555894fdf0, addr=151138272,
>      type=MMU_DATA_LOAD, type@entry=MMU_DATA_STORE, mmu_idx=6, memop=memop@entry=MO_8,
>      size=-1739692016, size@entry=151138272, probe=true, ra=0) at accel/tcg/cputlb.c:1251
> #5  0x0000555555e6eb0d in probe_access_internal (cpu=cpu@entry=0x55555894fdf0,
>      addr=addr@entry=151138272, fault_size=fault_size@entry=0,
>      access_type=access_type@entry=MMU_DATA_STORE, mmu_idx=<optimized out>,
>      nonfault=nonfault@entry=true, phost=0x7ffebeffc0a8, pfull=0x7ffebeffbfa0, retaddr=0,
>      check_mem_cbs=false) at accel/tcg/cputlb.c:1371
> #6  0x0000555555e70c84 in probe_access_full_mmu (env=0x5555589529b0, addr=addr@entry=151138272,
>      size=size@entry=0, access_type=access_type@entry=MMU_DATA_STORE, mmu_idx=<optimized out>,
>      phost=phost@entry=0x7ffebeffc0a8, pfull=0x7ffebeffbfa0) at accel/tcg/cputlb.c:1439
> #7  0x0000555555d497c9 in ptw_translate (inout=0x7ffebeffc090, addr=151138272)
>      at target/i386/tcg/system/excp_helper.c:68
> #8  0x0000555555d49988 in mmu_translate (env=env@entry=0x5555589529b0,
>      in=in@entry=0x7ffebeffc140, out=out@entry=0x7ffebeffc110, err=err@entry=0x7ffebeffc120,
>      ra=ra@entry=0) at target/i386/tcg/system/excp_helper.c:198
> #9  0x0000555555d4aece in get_physical_address (env=0x5555589529b0, addr=18446741874686299840,
>      access_type=MMU_DATA_LOAD, mmu_idx=4, out=0x7ffebeffc110, err=0x7ffebeffc120, ra=0)
>      at target/i386/tcg/system/excp_helper.c:597
> #10 x86_cpu_tlb_fill (cs=0x55555894fdf0, addr=18446741874686299840, size=<optimized out>,
>      access_type=MMU_DATA_LOAD, mmu_idx=4, probe=<optimized out>, retaddr=0)
>      at target/i386/tcg/system/excp_helper.c:617
> #11 0x0000555555e6e8cf in tlb_fill_align (cpu=0x55555894fdf0, addr=18446741874686299840,
>      type=type@entry=MMU_DATA_LOAD, mmu_idx=4, memop=MO_8, memop@entry=MO_32, size=-1739692016,
>      size@entry=3776, probe=false, ra=0) at accel/tcg/cputlb.c:1251
> #12 0x0000555555e6ed4d in mmu_lookup1 (cpu=cpu@entry=0x55555894fdf0,
>      data=data@entry=0x7ffebeffc310, memop=memop@entry=MO_32, mmu_idx=mmu_idx@entry=4,
>      access_type=access_type@entry=MMU_DATA_LOAD, ra=ra@entry=0) at accel/tcg/cputlb.c:1652
> #13 0x0000555555e6eea5 in mmu_lookup (cpu=cpu@entry=0x55555894fdf0,
>      addr=addr@entry=18446741874686299840, oi=oi@entry=36, ra=ra@entry=0,
>      type=type@entry=MMU_DATA_LOAD, l=l@entry=0x7ffebeffc310) at accel/tcg/cputlb.c:1755
> #14 0x0000555555e6f2f3 in do_ld4_mmu (cpu=cpu@entry=0x55555894fdf0,
>      addr=addr@entry=18446741874686299840, oi=oi@entry=36, ra=ra@entry=0,
>      access_type=access_type@entry=MMU_DATA_LOAD) at accel/tcg/cputlb.c:2364
> #15 0x0000555555e71dba in cpu_ldl_mmu (env=0x5555589529b0, addr=18446741874686299840, oi=36,
>      ra=0) at accel/tcg/ldst_common.c.inc:165
> #16 cpu_ldl_le_mmuidx_ra (env=env@entry=0x5555589529b0, addr=addr@entry=18446741874686299840,
>      mmu_idx=<optimized out>, ra=ra@entry=0) at accel/tcg/ldst_common.c.inc:308
> #17 0x0000555555db72da in do_interrupt64 (env=0x5555589529b0, intno=236, is_int=0, error_code=0,
>      next_eip=<optimized out>, is_hw=0) at target/i386/tcg/seg_helper.c:954
> #18 do_interrupt_all (cpu=cpu@entry=0x55555894fdf0, intno=236, is_int=is_int@entry=0,
>      error_code=error_code@entry=0, next_eip=next_eip@entry=0, is_hw=is_hw@entry=1)
>      at target/i386/tcg/seg_helper.c:1213
> #19 0x0000555555db884a in do_interrupt_x86_hardirq (env=env@entry=0x5555589529b0,
>      intno=<optimized out>, is_hw=is_hw@entry=1) at target/i386/tcg/seg_helper.c:1245
> #20 0x0000555555d4f06f in x86_cpu_exec_interrupt (cs=0x55555894fdf0,
>      interrupt_request=<optimized out>) at target/i386/tcg/system/seg_helper.c:209
> #21 0x0000555555e660ed in cpu_handle_interrupt (cpu=0x55555894fdf0, last_tb=<synthetic pointer>)
>      at accel/tcg/cpu-exec.c:851
> #22 cpu_exec_loop (cpu=cpu@entry=0x55555894fdf0, sc=sc@entry=0x7ffebeffc580)
>      at accel/tcg/cpu-exec.c:955
> #23 0x0000555555e663f1 in cpu_exec_setjmp (cpu=cpu@entry=0x55555894fdf0,
>      sc=sc@entry=0x7ffebeffc580) at accel/tcg/cpu-exec.c:1033
>      type=type@entry=MMU_DATA_LOAD, l=l@entry=0x7ffebeffc310) at accel/tcg/cputlb.c:1755
> #14 0x0000555555e6f2f3 in do_ld4_mmu (cpu=cpu@entry=0x55555894fdf0,
>      addr=addr@entry=18446741874686299840, oi=oi@entry=36, ra=ra@entry=0,
>      access_type=access_type@entry=MMU_DATA_LOAD) at accel/tcg/cputlb.c:2364
> #15 0x0000555555e71dba in cpu_ldl_mmu (env=0x5555589529b0, addr=18446741874686299840, oi=36,
>      ra=0) at accel/tcg/ldst_common.c.inc:165
> #16 cpu_ldl_le_mmuidx_ra (env=env@entry=0x5555589529b0, addr=addr@entry=18446741874686299840,
>      mmu_idx=<optimized out>, ra=ra@entry=0) at accel/tcg/ldst_common.c.inc:308
> #17 0x0000555555db72da in do_interrupt64 (env=0x5555589529b0, intno=236, is_int=0, error_code=0,
>      next_eip=<optimized out>, is_hw=0) at target/i386/tcg/seg_helper.c:954
> #18 do_interrupt_all (cpu=cpu@entry=0x55555894fdf0, intno=236, is_int=is_int@entry=0,
>      error_code=error_code@entry=0, next_eip=next_eip@entry=0, is_hw=is_hw@entry=1)
>      at target/i386/tcg/seg_helper.c:1213
> #19 0x0000555555db884a in do_interrupt_x86_hardirq (env=env@entry=0x5555589529b0,
>      intno=<optimized out>, is_hw=is_hw@entry=1) at target/i386/tcg/seg_helper.c:1245
> #20 0x0000555555d4f06f in x86_cpu_exec_interrupt (cs=0x55555894fdf0,
>      interrupt_request=<optimized out>) at target/i386/tcg/system/seg_helper.c:209
> #21 0x0000555555e660ed in cpu_handle_interrupt (cpu=0x55555894fdf0, last_tb=<synthetic pointer>)
>      at accel/tcg/cpu-exec.c:851
> #22 cpu_exec_loop (cpu=cpu@entry=0x55555894fdf0, sc=sc@entry=0x7ffebeffc580)
>      at accel/tcg/cpu-exec.c:955
> #23 0x0000555555e663f1 in cpu_exec_setjmp (cpu=cpu@entry=0x55555894fdf0,
>      sc=sc@entry=0x7ffebeffc580) at accel/tcg/cpu-exec.c:1033
> --Type <RET> for more, q to quit, c to continue without paging--
> #24 0x0000555555e66a5d in cpu_exec (cpu=cpu@entry=0x55555894fdf0) at accel/tcg/cpu-exec.c:1059
> #25 0x0000555555d2bdc7 in tcg_cpu_exec (cpu=cpu@entry=0x55555894fdf0)
>      at accel/tcg/tcg-accel-ops.c:80
> #26 0x0000555555d2c1c3 in mttcg_cpu_thread_fn (arg=arg@entry=0x55555894fdf0)
>      at accel/tcg/tcg-accel-ops-mttcg.c:94
> #27 0x0000555556056d90 in qemu_thread_start (args=0x5555589cdba0) at util/qemu-thread-posix.c:541
> #28 0x00007ffff60e0b7b in ?? () from /lib/x86_64-linux-gnu/libc.so.6
> #29 0x00007ffff615e7b8 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
> 
> 
> qemu-system-x86_64: ./include/exec/ram_addr.h:91: ramblock_ptr: Assertion `offset_in_ramblock(block, offset)' failed.
> 
> (gdb) bt
> #0  0x00007ffff6076507 in abort () from /lib/x86_64-linux-gnu/libc.so.6
> #1  0x00007ffff6076420 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
> #2  0x0000555555a047fa in ramblock_ptr (offset=281471527758833, block=<optimized out>)
>      at ./include/exec/ram_addr.h:91
> #3  0x0000555555a04c83 in ramblock_ptr (block=<optimized out>, offset=<optimized out>)
>      at system/physmem.c:2238
> #4  qemu_ram_ptr_length (lock=false, is_write=true, block=<optimized out>, addr=<optimized out>,
>      size=0x0) at system/physmem.c:2430
> #5  qemu_map_ram_ptr (ram_block=<optimized out>, addr=<optimized out>) at system/physmem.c:2443
> #6  0x0000555555e4af6b in memory_region_get_ram_ptr (mr=<optimized out>) at system/memory.c:2452
> #7  0x0000555555e6e024 in tlb_set_page_full (cpu=cpu@entry=0x5555589f9f50,
>      mmu_idx=mmu_idx@entry=4, addr=addr@entry=18446741874686296064,
>      full=full@entry=0x7ffebd7f90b0) at accel/tcg/cputlb.c:1065
> #8  0x0000555555e70a92 in tlb_set_page_with_attrs (cpu=cpu@entry=0x5555589f9f50,
>      addr=addr@entry=18446741874686296064, paddr=paddr@entry=206749696, attrs=...,
>      prot=<optimized out>, mmu_idx=mmu_idx@entry=4, size=4096) at accel/tcg/cputlb.c:1193
> #9  0x0000555555d4ae44 in x86_cpu_tlb_fill (cs=0x5555589f9f50, addr=18446741874686299840,
>      size=<optimized out>, access_type=MMU_DATA_LOAD, mmu_idx=4, probe=<optimized out>, retaddr=0)
>      at target/i386/tcg/system/excp_helper.c:624
> #10 0x0000555555e6e8cf in tlb_fill_align (cpu=0x5555589f9f50, addr=18446741874686299840,
>      type=type@entry=MMU_DATA_LOAD, mmu_idx=4, memop=MO_8, memop@entry=MO_32, size=-1115714056,
>      size@entry=3776, probe=false, ra=0) at accel/tcg/cputlb.c:1251
> #11 0x0000555555e6ed4d in mmu_lookup1 (cpu=cpu@entry=0x5555589f9f50,
>      data=data@entry=0x7ffebd7f9310, memop=memop@entry=MO_32, mmu_idx=mmu_idx@entry=4,
>      access_type=access_type@entry=MMU_DATA_LOAD, ra=ra@entry=0) at accel/tcg/cputlb.c:1652
> #12 0x0000555555e6eea5 in mmu_lookup (cpu=cpu@entry=0x5555589f9f50,
>      addr=addr@entry=18446741874686299840, oi=oi@entry=36, ra=ra@entry=0,
>      type=type@entry=MMU_DATA_LOAD, l=l@entry=0x7ffebd7f9310) at accel/tcg/cputlb.c:1755
> #13 0x0000555555e6f2f3 in do_ld4_mmu (cpu=cpu@entry=0x5555589f9f50,
>      addr=addr@entry=18446741874686299840, oi=oi@entry=36, ra=ra@entry=0,
>      access_type=access_type@entry=MMU_DATA_LOAD) at accel/tcg/cputlb.c:2364
> #14 0x0000555555e71dba in cpu_ldl_mmu (env=0x5555589fcb10, addr=18446741874686299840, oi=36,
>      ra=0) at accel/tcg/ldst_common.c.inc:165
> #15 cpu_ldl_le_mmuidx_ra (env=env@entry=0x5555589fcb10, addr=addr@entry=18446741874686299840,
>      mmu_idx=<optimized out>, ra=ra@entry=0) at accel/tcg/ldst_common.c.inc:308
> #16 0x0000555555db72da in do_interrupt64 (env=0x5555589fcb10, intno=236, is_int=0, error_code=0,
>      next_eip=<optimized out>, is_hw=0) at target/i386/tcg/seg_helper.c:954
> #17 do_interrupt_all (cpu=cpu@entry=0x5555589f9f50, intno=236, is_int=is_int@entry=0,
>      error_code=error_code@entry=0, next_eip=next_eip@entry=0, is_hw=is_hw@entry=1)
>      at target/i386/tcg/seg_helper.c:1213
> #18 0x0000555555db884a in do_interrupt_x86_hardirq (env=env@entry=0x5555589fcb10,
>      intno=<optimized out>, is_hw=is_hw@entry=1) at target/i386/tcg/seg_helper.c:1245
> #19 0x0000555555d4f06f in x86_cpu_exec_interrupt (cs=0x5555589f9f50,
>      interrupt_request=<optimized out>) at target/i386/tcg/system/seg_helper.c:209
> #20 0x0000555555e660ed in cpu_handle_interrupt (cpu=0x5555589f9f50, last_tb=<synthetic pointer>)
>      at accel/tcg/cpu-exec.c:851
> #21 cpu_exec_loop (cpu=cpu@entry=0x5555589f9f50, sc=sc@entry=0x7ffebd7f9580)
>      at accel/tcg/cpu-exec.c:955
> #22 0x0000555555e663f1 in cpu_exec_setjmp (cpu=cpu@entry=0x5555589f9f50,
>      sc=sc@entry=0x7ffebd7f9580) at accel/tcg/cpu-exec.c:1033
> #23 0x0000555555e66a5d in cpu_exec (cpu=cpu@entry=0x5555589f9f50) at accel/tcg/cpu-exec.c:1059
> #24 0x0000555555d2bdc7 in tcg_cpu_exec (cpu=cpu@entry=0x5555589f9f50)
>      at accel/tcg/tcg-accel-ops.c:80
> #25 0x0000555555d2c1c3 in mttcg_cpu_thread_fn (arg=arg@entry=0x5555589f9f50)
>      at accel/tcg/tcg-accel-ops-mttcg.c:94
> #26 0x0000555556056d90 in qemu_thread_start (args=0x55555856bf60) at util/qemu-thread-posix.c:541
> #27 0x00007ffff60e0b7b in ?? () from /lib/x86_64-linux-gnu/libc.so.6
> #28 0x00007ffff615e7b8 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
> 
> (gdb) frame 2
> #2  0x0000555555a047fa in ramblock_ptr (offset=281471527758833, block=<optimized out>)
>      at ./include/exec/ram_addr.h:91
> 91          assert(offset_in_ramblock(block, offset));
> 
> (gdb) l
> 86          return (b && b->host && offset < b->used_length) ? true : false;
> 87      }
> 88
> 89      static inline void *ramblock_ptr(RAMBlock *block, ram_addr_t offset)
> 90      {
> 91          assert(offset_in_ramblock(block, offset));
> 92          return (char *)block->host + offset;
> 93      }
> 94
> 95      static inline unsigned long int ramblock_recv_bitmap_offset(void *host_addr,
> 
> 
> [    9.439487] pci 0000:00:02.0: BAR 1 [io  0xc000-0xc03f]
> 
> Thread 65 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
> [Switching to Thread 0x7ffe9cff96c0 (LWP 15472)]
> phys_page_find (d=d@entry=0x7fff905ec880, addr=addr@entry=111288320) at system/physmem.c:337
> 337         if (section_covers_addr(&sections[lp.ptr], addr)) {
> (gdb) l
> 332             }
> 333             p = nodes[lp.ptr];
> 334             lp = p[(index >> (i * P_L2_BITS)) & (P_L2_SIZE - 1)];
> 335         }
> 336
> 337         if (section_covers_addr(&sections[lp.ptr], addr)) {
> 338             return &sections[lp.ptr];
> 339         } else {
> 340             return &sections[PHYS_SECTION_UNASSIGNED];
> 341         }
> (gdb)
> 
> 
> I was doing a bisection between 9.2.0 and 10.0.0, since we observed
> this issue happening wiht 10.0 but not - with 9.2.  So some of the
> above failures might be somewhere from the middle between 9.2 and
> 10.0.  However, I was able to trigger some of the failures with
> 9.2.0, though with much less probability.  And some can be triggered
> in current master too, with much better probability.
> 
> On my 4-core notebook, the above command line fails every 20..50 run.
> 
> I was never able to reproduce the assertion failure as shown in !1921.
> 
> As of now, this issue is hitting debian trixie, - in debci, when a
> package which creates a guest image tries to run qemu but in the
> debci environment there's no kvm available, so it resorts to tcg.
> 
> On IRC, Manos Pitsidianakis noted that he was debugging use-after-free
> with MemoryRegion recently, and posted a patch which can help a bit:
> https://people.linaro.org/~manos.pitsidianakis/backtrace.diff
> 
> I'm not sure where to go from here.
> 
> Just collecting everything we have now.
> 
> Thanks,
> 
> /mjt
>