Re: [PATCH v3] riscv: Fix a NULL pointer dereference in machine_kexec_prepare

Hi Pratyush,

On Thu, Jul 2, 2026 at 12:06 AM Pratyush Yadav <[email protected]> wrote:
>
> On Wed, Jul 01 2026, Tao Liu wrote:
>
> > Hi Jarkko,
> >
> > On Wed, Jul 1, 2026 at 3:50 PM Jarkko Sakkinen <[email protected]> wrote:
> >>
> >> On Wed, Jul 01, 2026 at 02:57:33PM +1200, Tao Liu wrote:
> >> > A NULL pointer dereference issue is noticed in riscv's machine_kexec_prepare,
> >> > where image->segment[i].buf might be NULL and copied unchecked.
> >> >
> >> > The NULL buf comes from security/integrity/ima/ima_kexec.c:
> >> > ima_add_kexec_buffer(), where kbuf is added by kexec_add_buffer(),
> >> > but kbuf.buffer is NULL
> >>
> >> This should have a proper call sequence. Now the root cause is
> >> obfuscated.
> >
> > Sure, I will attach the stack trace in v4. Here is the one:
> >
> > [   62.867540] kexec_file(Image): Loaded kernel at 0x80200000
> > bufsz=0x34ed800 memsz=0x35d0000
> > [   62.879983] Unable to handle kernel access to user memory without
> > uaccess routines at virtual address 0000000000000000
> > [   62.880736] Current kexec pgtable: 4K pagesize, 57-bit VAs,
> > pgdp=0x00000001062eb000
> > [   62.881185] [0000000000000000] pgd=00000000413b4401,
> > p4d=000000004151bc01, pud=00000000415b7801, pmd=0000000040af5801,
> > pte=0000000000000000
> > [   62.881969] Oops [#1]
> > [   62.882077] Modules linked in:
> > [   62.882717] CPU: 1 UID: 0 PID: 894 Comm: kexec Not tainted 7.1.1 #4
> > PREEMPT(lazy)
> > [   62.883037] Hardware name: QEMU QEMU Virtual Machine, BIOS
> > edk2-20260508-2.fc44 05/08/2026
> > [   62.883365] epc : __memcpy+0xd4/0xf8
> > [   62.883685]  ra : machine_kexec_prepare+0x8a/0x298
> > [   62.883914] epc : ffffffff81393ee8 ra : ffffffff800366ca sp :
> > ff20000004a83d10
> > [   62.884214]  gp : ffffffff83258db8 tp : ff6000008573db80 t0 :
> > ffffffff80033640
> > [   62.884433]  t1 : 2152ffffffffffc0 t2 : 0000000003000000 s0 :
> > ff20000004a83d80
> > [   62.884710]  s1 : 0000000000000000 a0 : ff20000004a83d10 a1 :
> > 0000000000000000
> > [   62.884987]  a2 : 0000000000000028 a3 : 0000000000000028 a4 :
> > 0000000000000000
> > [   62.885208]  a5 : 0000000000000000 a6 : 0000000104e33fff a7 :
> > 0000000000000000
> > [   62.885486]  s2 : ff60000082a35800 s3 : 0000000000000000 s4 :
> > 0000000000000010
> > [   62.885774]  s5 : 0000000000000028 s6 : ff20000004a83d10 s7 :
> > 0000000000000005
> > [   62.886005]  s8 : 00000000000000c0 s9 : 000000000ac0d220 s10:
> > ffffffff835420e8
> > [   62.886218]  s11: 0000000000000000 t3 : ffffff800000007c t4 :
> > ff1c000002138d00
> > [   62.886515]  t5 : ffffffffffffffff t6 : ff20000004a83d10 ssp :
> > 0000000000000000
> > [   62.886860] status: 0000000200000120 badaddr: 0000000000000000
> > cause: 000000000000000d
> > [   62.887162] [<ffffffff81393ee8>] __memcpy+0xd4/0xf8
> > [   62.887388] [<ffffffff801b253a>] __do_sys_kexec_file_load+0x1b2/0x338
> > [   62.887612] [<ffffffff801b26e4>] __riscv_sys_kexec_file_load+0x24/0x40
> > [   62.887855] [<ffffffff81395ea4>] do_trap_ecall_u+0x1a4/0x5a8
> > [   62.888134] [<ffffffff813a9eec>] handle_exception+0x16c/0x178
> > [   62.888445] Code: 7613 07f6 ca05 86b3 00c5 e7b3 01f5 8fd5 8b8d eb89
> > (4198) 0591
> > [   62.889223] ---[ end trace 0000000000000000 ]---
> > Segmentation fault         kexec -l /boot/vmlinuz-7.1.1
> > --initrd=/boot/initramfs-7.1.1.img --reuse-cmdline
> >
> > (gdb) p image->segment[0]
> > $3 = {{buf = 0x0, kbuf = 0x0}, bufsz = 0, mem = 10737586176, memsz = 4096}
> >
> > The buf = 0x0 and bufsz = 0 comes from ima_add_kexec_buffer(), thought
> > I'm not sure why it added a NULL segment, but it is no harm to add a
> > NULL checker here in case any other scenarios similar as IMA.
>
> From my reading of the IMA code, I think they do so because they don't
> want to add a buffer to the kimage just yet. This is because there can
> be IMA measurements between kexec load and kexec reboot. So they use
> kexec_add_buffer() to reserve the physical address space, then they vmap
> the destination pages in ima_kexec_post_load(), and then update the
> destination pages directly via a reboot notifier. So they do not have a
> buffer copied to the destination pages at the time of
> kexec_add_buffer().
>
> Now all that is fairly convoluted and not very obvious, but I am not
> sure if there is a better way of doing this off the top of my head.

Thank you very much for resolving my confusion! Now I understand it
much better...

Thanks,
Tao Liu

>
> >
> >>
> >> >
> >> > Fix this by simply adding a check before copy.
> >> >
> >> > Fixes: b7fb4d78a6ad ("RISC-V: use memcpy for kexec_file mode")
> >> > Acked-by: Baoquan He <[email protected]>
> >> > Acked-by: Pratyush Yadav <[email protected]>
> >> > Signed-off-by: Tao Liu <[email protected]>
> [...]
>
> --
> Regards,
> Pratyush Yadav
>