[DCCP] tfrc: Add protection against invalid parameters to TFRC routines

 1) For the forward X_calc lookup, it
    * protects effectively against RTT=0 (this case is possible), by
      returning the maximal lookup value instead of just setting it to 1
    * reformulates the array-bounds exceeded condition: this only happens
      if p is greater than 1E6 (due to the scaling)
    * the case of negative indices can now with certainty be excluded,
      since documentation shows that the formulas are within bounds
    * additional protection against p = 0 (would give divide-by-zero)

 2) For the reverse lookup, it warns against
    * protects against exceeding array bounds
    * now returns 0 if f(p) = 0, due to function definition
    * warns about minimal resolution error and returns the smallest table
      value instead of p=0 [this would mask congestion conditions]

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

1 files changed, 19 insertions, 14 deletions

diff --git a/net/dccp/ccids/lib/tfrc_equation.c b/net/dccp/ccids/lib/tfrc_equation.c
index 78bdf3489162..ef3233d45a61 100644
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -608,22 +608,19 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
         u32 f;
         u64 tmp1, tmp2;
 
+        /* check against invalid parameters and divide-by-zero   */
+        BUG_ON(p >  1000000);           /* p must not exceed 100%   */
+        BUG_ON(p == 0);                 /* f(0) = 0, divide by zero */
+        if (R == 0) {                   /* possible  divide by zero */
+                DCCP_CRIT("WARNING: RTT is 0, returning maximum X_calc.");
+                return ~0U;
+        }
+
         if (p < TFRC_CALC_X_SPLIT)                    /* 0      <= p <  0.05  */
                 index = (p / (TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE)) - 1;
         else                                          /* 0.05   <= p <= 1.00  */
                 index = (p / (1000000 / TFRC_CALC_X_ARRSIZE)) - 1;
 
-        if (index < 0)
-                /* p should be 0 unless there is a bug in my code */
-                index = 0;
-
-        if (R == 0) {
-                DCCP_WARN("RTT==0, setting to 1\n");
-                R = 1; /* RTT can't be zero or else divide by zero */
-        }
-
-        BUG_ON(index >= TFRC_CALC_X_ARRSIZE);
-
         if (p >= TFRC_CALC_X_SPLIT)
                 f = tfrc_calc_x_lookup[index][0];
         else
@@ -653,13 +650,21 @@ u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
         int ctr = 0;
         int small;
 
-        if (fvalue < tfrc_calc_x_lookup[0][1])
+        if (fvalue == 0)        /* f(p) = 0  whenever  p = 0 */
                 return 0;
 
+        /* Error cases. */
+        if (fvalue < tfrc_calc_x_lookup[0][1]) {
+                DCCP_WARN("fvalue %d smaller than resolution\n", fvalue);
+                return tfrc_calc_x_lookup[0][1];
+        }
+        if (fvalue > tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][0]) {
+                DCCP_WARN("fvalue %d exceeds bounds!\n", fvalue);
+                return 1000000;
+        }
+
         if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1])
                 small = 1;
-        else if (fvalue > tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][0])
-                return 1000000;
         else
                 small = 0;