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Backport 2.7: Fix buffer overflow in mbedtls_mpi_sub_abs negative case #4077

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Feb 2, 2021
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Backport 2.7: Fix buffer overflow in mbedtls_mpi_sub_abs negative case #4077

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b20bbd1
Add mpi_sub_abs negative tests with a larger-in-size second operand
gilles-peskine-arm Jan 27, 2021
54c3046
mbedtls_mpi_sub_abs: fix buffer overflow in error case
gilles-peskine-arm Jan 27, 2021
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7 changes: 7 additions & 0 deletions ChangeLog.d/mpi_sub_abs.txt
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
Security
* Fix a buffer overflow in mbedtls_mpi_sub_abs() when calculating
|A| - |B| where |B| is larger than |A| and has more limbs (so the
function should return MBEDTLS_ERR_MPI_NEGATIVE_VALUE). Only
applications calling mbedtls_mpi_sub_abs() directly are affected:
all calls inside the library were safe since this function is
only called with |A| >= |B|. Reported by Guido Vranken in #4042.
6 changes: 6 additions & 0 deletions library/bignum.c
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Expand Up @@ -1191,6 +1191,12 @@ int mbedtls_mpi_sub_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
for( n = B->n; n > 0; n-- )
if( B->p[n - 1] != 0 )
break;
if( n > A->n )
{
/* B >= (2^ciL)^n > A */
ret = MBEDTLS_ERR_MPI_NEGATIVE_VALUE;
goto cleanup;
}

carry = mpi_sub_hlp( n, X->p, B->p );
if( carry != 0 )
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20 changes: 16 additions & 4 deletions tests/suites/test_suite_mpi.data
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Expand Up @@ -466,18 +466,30 @@ mbedtls_mpi_add_int:10:"20395687835640197740576586692903457728019399331434826309
Test mbedtls_mpi_add_int #2
mbedtls_mpi_add_int:10:"2039568783564019774057658669290345772801939933143482630947726464532830627227012776329":-9871232:10:"2039568783564019774057658669290345772801939933143482630947726464532830627227002905097"

Base test mbedtls_mpi_sub_abs #1 (Test with larger second input)
Base test mbedtls_mpi_sub_abs #1 (|B| > |A|)
mbedtls_mpi_sub_abs:10:"5":10:"7":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE

Base test mbedtls_mpi_sub_abs #2 (Test with larger second input)
Base test mbedtls_mpi_sub_abs #2 (|B| > |A|)
mbedtls_mpi_sub_abs:10:"-5":10:"-7":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE

Base test mbedtls_mpi_sub_abs #3 (Test with larger second input)
Base test mbedtls_mpi_sub_abs #3 (|B| > |A|)
mbedtls_mpi_sub_abs:10:"-5":10:"7":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE

Base test mbedtls_mpi_sub_abs #4 (Test with larger second input)
Base test mbedtls_mpi_sub_abs #4 (|B| > |A|)
mbedtls_mpi_sub_abs:10:"5":10:"-7":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE

Base test mbedtls_mpi_sub_abs #1 (|B| >> |A| with more limbs)
mbedtls_mpi_sub_abs:10:"5":16:"123456789abcdef01":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE

Base test mbedtls_mpi_sub_abs #2 (|B| >> |A| with more limbs)
mbedtls_mpi_sub_abs:10:"-5":16:"-123456789abcdef01":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE

Base test mbedtls_mpi_sub_abs #3 (|B| >> |A| with more limbs)
mbedtls_mpi_sub_abs:10:"-5":16:"123456789abcdef01":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE

Base test mbedtls_mpi_sub_abs #4 (|B| >> |A| with more limbs)
mbedtls_mpi_sub_abs:10:"5":16:"-123456789abcdef01":10:"0":MBEDTLS_ERR_MPI_NEGATIVE_VALUE

Base test mbedtls_mpi_sub_abs #1
mbedtls_mpi_sub_abs:10:"7":10:"5":10:"2":0

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