Commit message: Fix integer overflow in STRALGO LCS (CVE-2021-29477)

An integer overflow bug in Redis version 6.0 or newer could be exploited using
the STRALGO LCS command to corrupt the heap and potentially result with remote
code execution.

(cherry picked from commit f0c5f920d0f88bd8aa376a2c05af4902789d1ef9)
CVE: CVE-2021-29477
CWE: CWE-190
Vulnerability description: Redis is an open source (BSD licensed), in-memory data structure store, used as a database, cache, and message broker. An integer overflow bug in Redis version 6.0 or newer could be exploited using the `STRALGO LCS` command to corrupt the heap and potentially result with remote code execution. The problem is fixed in version 6.2.3 and 6.0.13. An additional workaround to mitigate the problem without patching the redis-server executable is to use ACL configuration to prevent clients from using the `STRALGO LCS` command.

Project: redis
File: t_string.c

void stralgoLCS(client *c) {
    uint32_t i, j;
    long long minmatchlen = 0;
    sds a = NULL, b = NULL;
    int getlen = 0, getidx = 0, withmatchlen = 0;
    robj *obja = NULL, *objb = NULL;

    for (j = 2; j < (uint32_t)c->argc; j++) {
        char *opt = c->argv[j]->ptr;
        int moreargs = (c->argc-1) - j;

        if (!strcasecmp(opt,"IDX")) {
            getidx = 1;
        } else if (!strcasecmp(opt,"LEN")) {
            getlen = 1;
        } else if (!strcasecmp(opt,"WITHMATCHLEN")) {
            withmatchlen = 1;
        } else if (!strcasecmp(opt,"MINMATCHLEN") && moreargs) {
            if (getLongLongFromObjectOrReply(c,c->argv[j+1],&minmatchlen,NULL)
                != C_OK) goto cleanup;
            if (minmatchlen < 0) minmatchlen = 0;
            j++;
        } else if (!strcasecmp(opt,"STRINGS") && moreargs > 1) {
            if (a != NULL) {
                addReplyError(c,"Either use STRINGS or KEYS");
                goto cleanup;
            }
            a = c->argv[j+1]->ptr;
            b = c->argv[j+2]->ptr;
            j += 2;
        } else if (!strcasecmp(opt,"KEYS") && moreargs > 1) {
            if (a != NULL) {
                addReplyError(c,"Either use STRINGS or KEYS");
                goto cleanup;
            }
            obja = lookupKeyRead(c->db,c->argv[j+1]);
            objb = lookupKeyRead(c->db,c->argv[j+2]);
            if ((obja && obja->type != OBJ_STRING) ||
                (objb && objb->type != OBJ_STRING))
            {
                addReplyError(c,
                    "The specified keys must contain string values");
                /* Don't cleanup the objects, we need to do that
                 * only after calling getDecodedObject(). */
                obja = NULL;
                objb = NULL;
                goto cleanup;
            }
            obja = obja ? getDecodedObject(obja) : createStringObject("",0);
            objb = objb ? getDecodedObject(objb) : createStringObject("",0);
            a = obja->ptr;
            b = objb->ptr;
            j += 2;
        } else {
            addReplyErrorObject(c,shared.syntaxerr);
            goto cleanup;
        }
    }

    /* Complain if the user passed ambiguous parameters. */
    if (a == NULL) {
        addReplyError(c,"Please specify two strings: "
                        "STRINGS or KEYS options are mandatory");
        goto cleanup;
    } else if (getlen && getidx) {
        addReplyError(c,
            "If you want both the length and indexes, please "
            "just use IDX.");
        goto cleanup;
    }

    /* Compute the LCS using the vanilla dynamic programming technique of
     * building a table of LCS(x,y) substrings. */
    uint32_t alen = sdslen(a);
    uint32_t blen = sdslen(b);

    /* Setup an uint32_t array to store at LCS[i,j] the length of the
     * LCS A0..i-1, B0..j-1. Note that we have a linear array here, so
     * we index it as LCS[j+(blen+1)*j] */
    uint32_t *lcs = zmalloc((alen+1)*(blen+1)*sizeof(uint32_t));
    #define LCS(A,B) lcs[(B)+((A)*(blen+1))]

    /* Start building the LCS table. */
    for (uint32_t i = 0; i <= alen; i++) {
        for (uint32_t j = 0; j <= blen; j++) {
            if (i == 0 || j == 0) {
                /* If one substring has length of zero, the
                 * LCS length is zero. */
                LCS(i,j) = 0;
            } else if (a[i-1] == b[j-1]) {
                /* The len LCS (and the LCS itself) of two
                 * sequences with the same final character, is the
                 * LCS of the two sequences without the last char
                 * plus that last char. */
                LCS(i,j) = LCS(i-1,j-1)+1;
            } else {
                /* If the last character is different, take the longest
                 * between the LCS of the first string and the second
                 * minus the last char, and the reverse. */
                uint32_t lcs1 = LCS(i-1,j);
                uint32_t lcs2 = LCS(i,j-1);
                LCS(i,j) = lcs1 > lcs2 ? lcs1 : lcs2;
            }
        }
    }

    /* Store the actual LCS string in "result" if needed. We create
     * it backward, but the length is already known, we store it into idx. */
    uint32_t idx = LCS(alen,blen);
    sds result = NULL;        /* Resulting LCS string. */
    void *arraylenptr = NULL; /* Deffered length of the array for IDX. */
    uint32_t arange_start = alen, /* alen signals that values are not set. */
             arange_end = 0,
             brange_start = 0,
             brange_end = 0;

    /* Do we need to compute the actual LCS string? Allocate it in that case. */
    int computelcs = getidx || !getlen;
    if (computelcs) result = sdsnewlen(SDS_NOINIT,idx);

    /* Start with a deferred array if we have to emit the ranges. */
    uint32_t arraylen = 0;  /* Number of ranges emitted in the array. */
    if (getidx) {
        addReplyMapLen(c,2);
        addReplyBulkCString(c,"matches");
        arraylenptr = addReplyDeferredLen(c);
    }

    i = alen, j = blen;
    while (computelcs && i > 0 && j > 0) {
        int emit_range = 0;
        if (a[i-1] == b[j-1]) {
            /* If there is a match, store the character and reduce
             * the indexes to look for a new match. */
            result[idx-1] = a[i-1];

            /* Track the current range. */
            if (arange_start == alen) {
                arange_start = i-1;
                arange_end = i-1;
                brange_start = j-1;
                brange_end = j-1;
            } else {
                /* Let's see if we can extend the range backward since
                 * it is contiguous. */
                if (arange_start == i && brange_start == j) {
                    arange_start--;
                    brange_start--;
                } else {
                    emit_range = 1;
                }
            }
            /* Emit the range if we matched with the first byte of
             * one of the two strings. We'll exit the loop ASAP. */
            if (arange_start == 0 || brange_start == 0) emit_range = 1;
            idx--; i--; j--;
        } else {
            /* Otherwise reduce i and j depending on the largest
             * LCS between, to understand what direction we need to go. */
            uint32_t lcs1 = LCS(i-1,j);
            uint32_t lcs2 = LCS(i,j-1);
            if (lcs1 > lcs2)
                i--;
            else
                j--;
            if (arange_start != alen) emit_range = 1;
        }

        /* Emit the current range if needed. */
        uint32_t match_len = arange_end - arange_start + 1;
        if (emit_range) {
            if (minmatchlen == 0 || match_len >= minmatchlen) {
                if (arraylenptr) {
                    addReplyArrayLen(c,2+withmatchlen);
                    addReplyArrayLen(c,2);
                    addReplyLongLong(c,arange_start);
                    addReplyLongLong(c,arange_end);
                    addReplyArrayLen(c,2);
                    addReplyLongLong(c,brange_start);
                    addReplyLongLong(c,brange_end);
                    if (withmatchlen) addReplyLongLong(c,match_len);
                    arraylen++;
                }
            }
            arange_start = alen; /* Restart at the next match. */
        }
    }

    /* Signal modified key, increment dirty, ... */

    /* Reply depending on the given options. */
    if (arraylenptr) {
        addReplyBulkCString(c,"len");
        addReplyLongLong(c,LCS(alen,blen));
        setDeferredArrayLen(c,arraylenptr,arraylen);
    } else if (getlen) {
        addReplyLongLong(c,LCS(alen,blen));
    } else {
        addReplyBulkSds(c,result);
        result = NULL;
    }

    /* Cleanup. */
    sdsfree(result);
    zfree(lcs);

cleanup:
    if (obja) decrRefCount(obja);
    if (objb) decrRefCount(objb);
    return;
}