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    SubjectRe: ESP corruption bug - what CPUs are affected?
    On Friday 17 September 2004 21:13, Stas Sergeev wrote:
    > Hello.
    > Denis Vlasenko wrote:
    > >> Which means that the higher word of ESP gets
    > >> trashed. This bug beats dosemu rather badly,
    > >> but there seem to be not much info about that
    > >> bug available on the net.
    > >
    > > Well. Strictly speaking it's not a bug.
    > > Code executes as it should. IRET returns to the correct
    > > location. Upper 16 bits of ESP do not affect execution
    > > in this case.
    > Unfortunately this is not true. Mainly because
    > the code is still 32bit, so when is operates with
    > the stack pointer directly, it gets ESP, not SP.
    > Here are a few examples from the real DOS progs:
    > ---
    > frstor [esp] <--crash
    > ---
    > push ebp
    > mov ebp,esp
    > mov edx,[ebp+0x8] <--crash
    > ---

    Hmmm. Let me dust off my i386 knwoledge.

    This is what happens: CPU is doing IRET in 32bit mode.
    Stack layout:

    +20 (empty) <--- SS:ESP
    +16 old_CS
    +12 old_EIP
    +8 old_EFLAGS
    +4 old_SS
    +0 old_ESP

    If old_SS references 'small' data segment (16-bit one),
    processor does not restore ESP from old_ESP.
    It restores lower 16 bits only. Upper 16 bits are filled
    with garbage (or just not modified at all?).
    This works fine because processor uses SP, not ESP,
    for subsequent push/pop/call/ret operations.
    But if code uses full ESP, thinking that upper 16 bits are zero,
    it will crash badly. Correct me if I'm wrong.

    > and there are much more. "mov ebp,esp" is just a
    > standard thing, and then the ebp is being used to
    > access the locals and function arguments. Why do
    > they use the 16bit stack in that case, is another
    > question. Unfortunately some of them do.

    Hmm. I think you need to *emulate* this IRET.
    Is this IRET happens in kernel or in dosemu code?

    > > # gcc -O2 stk.c
    > > # ./a.out
    > > sp=0xbffffb30, ss=0x7b
    > > In sighandler: esp=bffffb10
    > > Now sp=0xccf1fb10, ss=0x7f
    > > Esp changed, strange...
    > That's strange indeed! Apparently your ESP value,
    > 0xccf1fb10, is greater than 0xc0000000, in which
    > case my program should just write "BUG!", but for
    > you it doesn't. Haven't you altered the code
    > somehow?

    No, I didn't alter anything. Strange indeed.

    > Why I compare it with 0xc0000000, is
    > because ESP has the higher word of a kernel's stack
    > address, so it should be above the TASK_SIZE when
    > corrupted.

    Even more strange! Now I did modify the code (attached),
    and this is what I've got:

    # gcc -o stk2 -O2 stk2.c
    # ./stk2
    sp=0xbffffb30, ss=0x7b
    In sighandler: esp=bffffb10
    Now sp=cd79fb10 <=========
    Now sp=bffffb50 <========= !?
    Now sp=bffffb50, ss=007f
    Now sp=bffffb50
    Now sp=bffffb50
    Now sp=bffffb50
    Esp changed, strange...

    I am thoroughly confused now. Two back-to-back
    'printf("Now sp=%08lx\n", new_esp);'
    gave different result. How this can happen??

    Corresponding gcc -S code. new_esp is at -180(%ebp):

    movw -182(%ebp), %ss
    movw -170(%ebp), %ss
    movl %esp, -180(%ebp)
    movl -176(%ebp), %esp

    addl $40, %esp
    pushl -180(%ebp)
    pushl $.LC4
    call printf
    popl %eax
    popl %edx
    pushl -180(%ebp)
    pushl $.LC4
    call printf
    addl $12, %esp
    movzwl -182(%ebp), %eax
    pushl %eax
    pushl -180(%ebp)
    pushl $.LC5
    call printf
    popl %ecx
    popl %ebx
    pushl -180(%ebp)
    pushl $.LC4
    call printf
    popl %eax
    popl %edx

    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    #include <signal.h>
    #include <linux/unistd.h>
    #include <asm/ldt.h>
    #include <asm/ucontext.h>

    unsigned char stack[65536];

    void my_segv(int signum, siginfo_t *info, void *c)
    struct sigcontext *ctx = &((struct ucontext *)c)->uc_mcontext;
    printf("In sighandler: esp=%lx\n", ctx->esp);
    /* Skip HLT */

    _syscall3(int, modify_ldt, int, func, void *, ptr, unsigned long, bytecount);

    static int set_ldt_entry(int entry, unsigned long base, unsigned int limit,
    int seg_32bit_flag, int contents, int read_only_flag,
    int limit_in_pages_flag, int seg_not_present, int useable)
    struct modify_ldt_ldt_s ldt_info;
    ldt_info.entry_number = entry;
    ldt_info.base_addr = base;
    ldt_info.limit = limit;
    ldt_info.seg_32bit = seg_32bit_flag;
    ldt_info.contents = contents;
    ldt_info.read_exec_only = read_only_flag;
    ldt_info.limit_in_pages = limit_in_pages_flag;
    ldt_info.seg_not_present = seg_not_present;
    ldt_info.useable = useable;

    return modify_ldt(1, &ldt_info, sizeof(ldt_info));

    int main(int argc, char *argv[])
    unsigned short _ss, new_ss;
    unsigned long _esp, new_esp;
    int is32 = 0;
    stack_t sig_stack;
    struct sigaction sa;

    if (argc > 1 && !strncmp(argv[1], "32", 3))
    is32 = 1;

    sig_stack.ss_sp = stack;
    sig_stack.ss_flags = 0;
    sig_stack.ss_size = sizeof(stack);
    if (sigaltstack(&sig_stack, NULL)) {
    return 1;

    sa.sa_sigaction = my_segv;
    sa.sa_flags = SA_ONSTACK | SA_SIGINFO;
    sigaction(SIGSEGV, &sa, NULL);

    /* Get SS, ESP */
    asm volatile(
    "movw %%ss, %0\n"
    "movl %%esp, %1\n"
    :"=m"(_ss), "=m"(_esp)
    printf("sp=%#lx, ss=%#hx\n", _esp, _ss);

    /* Force to LDT */
    new_ss = _ss | 4;
    /* Create the LDT entry */
    set_ldt_entry(new_ss >> 3, 0, 0xbffff, is32, MODIFY_LDT_CONTENTS_DATA, 0, 1, 0, 0);

    /* Do the trick... Switch stack and then switch context. */
    asm volatile(
    "movw %3, %%ss\n" /* Load our LDT selector to SS */
    "hlt\n" /* Force the context switch */
    "movw %1, %%ss\n" /* Restore SS ASAP */
    "movl %%esp, %0\n" /* Get new ESP */
    "movl %2, %%esp\n" /* Restore ESP */
    : "m"(_ss), "m"(_esp), "m"(new_ss)
    printf("Now sp=%08lx\n", new_esp);
    printf("Now sp=%08lx\n", new_esp);
    printf("Now sp=%08lx, ss=%04hx\n", new_esp, new_ss);
    printf("Now sp=%08lx\n", new_esp);
    printf("Now sp=%08lx\n", new_esp);

    /* See what we've got... */
    if (new_esp > 0xc0000000) {
    printf("Now sp=%08lx\n", new_esp);
    } else if (new_esp != _esp) {
    printf("Now sp=%08lx\n", new_esp);
    printf("Esp changed, strange...\n");
    } else {
    printf("Now sp=%08lx\n", new_esp);
    printf("No bug here! What CPU is this?\n");
    if (!is32)
    system("cat /proc/cpuinfo");
    return 0;
     \ /
      Last update: 2005-03-22 14:06    [W:0.033 / U:9.260 seconds]
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